But even if life on Earth came from somewhere else...
It had to start somewhere, so the point still stands.
As for how life got going on this planet, why couldn't it be from more
than one source? A combo of dropping in from space on a Mars rock or
a comet, lightning striking a pool of water full of organic molecules,
clay formations turning complex, and hydrothermal vents in the deep
ocean producing materials for life might have given life enough odds to
actually make it happen here.
Perhaps this is what most planets need to have life: More than one
way/source to make it happen.