Even if panspermia is the leading theory for life on Earth, abiogenesis had to have happened somewhere at some point before.

I haven't had to link this in a long time. Here is the link to the relevant FAQ topic about abiogenesis from the talk.origins Usenet compilation. If you're honestly curious about the real statistics, that's a start. The cited works are obviously old but the science hasn't changed, if anything we've learned more.

Usually the strawman against abiogenesis is that a simple bacterium or virus can't just appear from nowhere, which of course is true but isn't what the science of the beginnings of life even remotely suggests. The opposite is actually true, in a world where there are no higher life forms to compete with we'd probably see all sorts of complex combinations of chemicals that eventually run across a replication process. This is the answer to OP's question, once higher life develops, the basic chemical replicators can't compete anymore. Or get absorbed into a symbiosis, as what seems to be the case with the mitochondria.

With the right conditions on other worlds (not necessarily only what Earth was like) simple life forms may be very common. We certainly now know just from recent sampling that there are planets everywhere.