Dumb question. Why can’t we focus on a single exoplanet, look for mountains, grass, buildings? Why am I so stupid but isn’t this the obvious thing to do?
There is a fundamental physics limit at play here: the diffraction limit is linear with the aperture diameter and gives an upper bound on the resolution of a telescope. Having a longer exposure doesn't help - that's for resolving very faint objects (more light collected -> higher signal-to-noise). To resolve a building-sized object on an exoplanet, regardless of its intensity, we'd need a telescope the size of the solar system. There are some proposals to use the gravitational lensing of our sun to create such a telescope, but those projects are decades at least from implementation.