Why are most discovered exoplanets heavier than Earth? Looking at all discovered exoplanets (4393 exoplanets), I found than only 17 of them (less than one percent!) have masses less or equal to Earth's mass. Why so?

Is it because it is very difficult to discover an exoplanet of a low mass?
Is it because of the mass distribution, so that Earth-mass planets are very rare?
Is it because of the some other physical limitations?

According to Wikipedia:

The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.

From another article:

A dwarf planet, by definition, is not massive enough to have gravitationally cleared its neighbouring region of planetesimals: it is not known quite how large a planet must be before it can effectively clear its neighbourhood, but one tenth of the Earth's mass is certainly sufficient.

So, where are all these planets that are lighter than Earth? Personally, I suspect that it's very difficult to detect these (relatively) low-mass planets. If so, are there any theoretical limitations that prevent formation of low-mass planets?
Note 1: most of the planets (around 70%) from the mentioned catalog do not have masses (i.e. there's no estimate for the mass of a planet). Most of the rest have $\sin i$ mass estimates. That might be one of the reasons.
There are a number of methods of detecting exoplanets, but all of them favour detection of larger planets over smaller ones, albeit for slightly different definitions of large:

Radial velocity measurement — this detects the small movement of the star towards and away from us as the planet and the star orbit their mutual barycenter. This movement is fastest when the planet is massive (so the barycenter is further from the center of the star) and close to the star (so the orbital velocity is highest). I also needs the planet's orbit not to be &quot;face-on&quot; to the Earth. This method produces measurements for the $mass\times \sin(i)$ since a more massive planet in a less inclined orbit produces the same motion as a less massive planet in a more inclined orbit

Transverse displacement — this detects the small movement of the star from side to side (against the background of distant stars) as the planet and the star orbit their mutual barycenter. The displacement is largest when the planet is massive and far from the star (although distant planet require observation over a long period of time).  It works best on stars close to us.

Transit — this detects the tiny reduction in the brightness of the star when the planet moves between us. It is more likely to detect large planets, and more likely to notice if the orbital period of the planet is fairly small.