Why are we so quick to proclaim the geological inhospitability of other planets based on extremely minimal information? Now, I'm not questioning claims that any planet in the Solar System is atmospherically or externally inhospitable, but claims made that other planets have geology that is extremely hostile to colonization (or even, in the case of Venus, further exploration).
Some of these claims are:


Mars's core is frozen.
Mars has no recent volcanic activity.
There are no large deposits of carbonates or other carbonaceous minerals/materials on Mars.
The total quantity of water on Mars is less than 1% that on Earth.
A perchlorate layer covers the entire Martian surface.
The soil of the Moon, Mars, and Venus would need to be massively modified to support plant life.
Venus was volcanically resurfaced in its entirety.
There are never any breaks in Venus's cloud layer, no matter how small or infrequent.


Judgements like this seem very premature given our lack of hard data, especially that of direct measurements of rock strata. For instance, about the first claim, only one functional siesmometer has been sent to the surface, which has only been there for a year and lacks any impact tests, and the only instrument sent to measure internal heat has so far malfunctioned. The 2nd and 7th claims are almost exclusively based on crater counts, which can give tremendously different age values depending on the expected impact rate. The 3rd is based exclusively on remote sensing methods unable to identify underlying strata, the 5th is based on a handful of landings, and the 6th has been called into question by research old and new. For the 8th, no modern space mission has even carried the instrumentation required to confirm this (that is, a high-resolution visible-light camera). All of these seem only possible to confirm or deny with much more extensive exploration, especially on and under the ground.

So, then, what am I missing, if anything? Why is it acceptable to make judgements like these off the information we have? I mean, imagine if a textbook of Earth's geology was writen solely on the observations of a handful of satellites as well as 9 pseudo-randomly-dispersed geologists allowed only a couple tens of kilograms of shallowly-penetrating tools and analysis devices (some defective) each, of which 5 of them are buried up to their knees in rock.
It is not simple to address all these points since they are simple expressions for complex processes.

1.) Mars's core is frozen

Translating frozen as solid. It is still unclear if Mars' core is solid or liquid, see @userLTK's answer: it is curently seen as being viscous. A liquid core could help maintaining a magnetic field. Mars has a very weak magnetic field that does not shield it like the earth's magnetic field does. This is adverse to the development of complex organic molecules and the basics of simple life. Maybe InSight brings us more insight into this one.

2.) Mars has no recent volcanic activity.

That appears to be incorrect. Mars has no plate tectonics that offer cycles (e.g.carbon). One can speculate that tectonis are helpful for a biologic evolution and complex life, offering renewal of needed elements, thermostats, being helpful in case of planetwide overfreezing, ... but this is debated and speculative.

There is possible polar volcanism and recent lava flows in the calderas of the Tharsis Montes (Olympus Mons and neighbours).

3.) There are no large deposits of carbonates or other carbonaceous minerals/materials on Mars.

There are carbonates, but not organic. That means that in the past organisms have not built up sequences of sediments like on earth. And it is true that remote sensing will not suffice for a qualified final judgement. But there is nothing more to do atm.

4.) The total quantity of water on Mars is less than 1% that on Earth.

Unclear, but probably not totally nonsensical. Though there is water, or rather brines. Water isn't exactly rare in the solar system. Mars probably had a water ocean early, but may have lost it qickly to space. Otoh it can be debated if amounts of water have been sequestered in the Martian mantle. This water could sum up to several meters worth of water coverage.

5.) A perchlorate layer covers the entire Martian surface.

Exaggerated, but perchlorates were found in samples of Marsian regolith and under UV radiation result in a bacteriocidal environment. No potatoes for Marc ...

6.) The soil of the Moon, Mars, and Venus would need to be massively modified to support plant life.

Well, that is indeed a belittlement. It is far beyond our capbilities to mobilize the necessary masses and energies. One could imagine greenhouses; and test runs have been/are being made in Antarctica. But who wants greenhouse stuff every day ;-). There is this study concluding that even if we could mobilize all the CO2 in all available reservoirs including the upper crust of Mars, that would only rise the atmospheric pressure by a few percent. Not enough to significally warm the atmosphere or walk around without a pressure suit (oxygene supply allways needed anyway).

7.) Venus was volcanically resurfaced in its entirety.

I have no idea, but so are large parts of Earth, constantly actually. But on the Venusian surface there are ~450°C and 95bar atmospheric pressure. The atmosphere is almost pure CO2. Excluding life until further notice is a valid guess.

8.) There are never any breaks in Venus's cloud layer, no matter how small or infrequent.

Ok.

We could address these points one by one and search for proper publications of data from spacecraft, remote sensing and rovers and apply the principle of uniformity (that's what NASA does, but don't use it as a religion ...) and discuss further. But this has all been done multiple times. All in all, for Venus the judgement "is dead" is pretty natural, given the conditions. Mars may have seen microbial life in its beginning, let's say its first few hundred million years (speculative !), but today this is unlikely, though not impossible. More data needed, as you say.