Until they do. Eventually bugs and diseases would kill off huge portions of them. Trees would still be around, but it'd definitely be a different mix than we have now. Right now every ash tree in my state is either dead or dying due to beetles, all the chestnuts died when I was a kid, the native oaks mostly only live where they've been planted because anytime one dies a faster growing maple replaces it.
In a dense cities, the trees you see now that were allowed to grow to maturity were mostly likely planted by a human, with some exceptions of course. Any trees that began to grow organically were probably cut or eliminated.
Until they do. Eventually bugs and diseases would kill off huge portions of them. Trees would still be around, but it'd definitely be a different mix than we have now. Right now every ash tree in my state is either dead or dying due to beetles, all the chestnuts died when I was a kid, the native oaks mostly only live where they've been planted because anytime one dies a faster growing maple replaces it.