If trees don't mate with a partner but just drop their seed and grow anew, why haven't like a billion different trees evolved from a natural selection with no real threats or sexual selection?
Why is there a few distinctly separate species?

I assume that shitty trees slowly disappeared because other trees are just better (more/better chloroplasts, physically more propitious to hogging sunlight and water). Have you tried growing something next to eucalypti?
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Also they might be worse at spreading their seeds

Yes but isn't it right that even shitty mutations would make it through their life cycle, including reproduction

Should there really not be more trees than the few there are for each climate?
Seems species of animals and insects suffer from natural and sexual selection to a greater extent and there's still more kinds of animals than trees which is suspicious

in additional to what pusga said i might add that trees themselfs could be dangerous to other trees as some use chemical attacks to kill any other competitor for water and light, and lets not forget that tree's seeds(and trees themselfs) are the main nutritions for alot of animals and insects which can cause alot of trees to die or atleast not be able to reproduce, and so only the trees that adapted by either becoming toxic or producing more seeds survived.

trees dont multiply... theyre just there....