You make a good point, why wouldn’t dark matter form it’s own stars and solar systems? There is certainly a tendency for normal matter to clump up and form huge clumps.
However, as to your first question. If dark matter only interacts gravitationally, then, if was not originally graviationally bound to the earth, it would gain enough kinetic energy as it fell towards (or even through!) the earth to have escape velocity from the earth as it passes through it and out the other side! The interaction of the two bodies (earth and dark matter) might deflect them, but there would be not net exchange of energy.
Dark matter from what I understand acts as an anti-gravitional force, causing matter to push apart rather than accumulate as in the case of gravity, which would explain why it doesn’t accumulate and form dark matter galaxies and so forth.
Scientists theorized it as an explanation for why the universe appears to be ramping up its expansion instead of collapsing back in on itself.