There is nothing stopping you from using other app stores on android. Amazon has their own (amazon underground) and samsung have their own (galaxy store). There are others out there, too!
Can you say what resources company invested to get listed on other stores? I'd expect it to be rather simple and would hope the tiny earnings would still pay for itself. But even for strategical reasons, I don't understand why even Google-competitors like Microsoft Office don't offer an optional PlayStore free download. At least for those not integrating Google services... and most of the apk's i got from "other" source did indeed work fine. But maybe there is just no way to avoid a tragedy of the commons without regulation.