Why don't we use amateur astronomers' telescopes to create a huge interferometer? Some telescopes in space have been proposed to function as an interferometer. 
Being placed several hundreds or thousands of kilometers apart they could actual image exoplanets directly. To my knowledge none of these space telescopes have yet been deployed.
So I was wondering why we don't connect all the several thousand amateur telescopes on Earth together to create a huge interferometer?
That sure would require some efforts (especially software to coordinate it all) but compared to sending telescopes into space it seems outright simple.
Are amateur telescopes simply not strong enough, even when linked together? What would be the best we could achieve?
You can't make an interferometer by combining photographs. You need to combine the light sources so that the light waves can &quot;interfere&quot;, which means you need to have not only the intensity of the light, but also the phase of the wavelength. A CCD only detects the intensity.
You also need to position your telescopes to sub-wavelength accuracy. For Radio telescopes, with long wavelengths, it is possible to detect and then combine the radio signals from several telescopes electronically. That's not possible with light. Visible wavelength interferometers capture the light and combine it as light. It requires very high precision.
Finally, we still have the atmosphere to contend with. Amateur observers know that, beyond a certain size (about 10in), larger telescopes don't have greater resolution, because of the blurring due to the atmosphere1. Bigger mirrors mean more light, mean dimmer objects can be seen, but no more detail is visible. There are ways of getting round this, (Lucky imaging) but these are not suited to interferometery.
So since placing amateur telescopes at nanometer accurate positions, and linking their output with single strand optical fibre is beyond the budget of amateurs, and since the results would be limited by the distortion of the atmosphere, this is not normally done.
1Large observatories like the Very Large Telescope use Adaptive optics to obtain dramatically improved resolution through Earth's atmosphere but that kind of technology is sophisticated and out of reach of amateur instruments.