I'm going to use an analogy from my hobby: because internal software is like shop furniture in a woodworking shop. Those things are mostly used in the production of other things that make the company revenue. It does its job, has room for many improvements, but isn't something you'd go out and sell as a product. It might be built from scraps, using crappy lumber and pieced together with whatever fasteners were around…
There are other reasons too: * Time pressure - OSS is rarely under deadline pressure so the temptation to half ass it is mitigated. * Survivorship bias - there's actually a lot of OSS shit out there too but it is much easier to ignore coz nobody is gonna force you to use it.
You could be comparing the work of hundreds of dedicated engineers for over a decade to something someone threw together in an afternoon, that was never intended for mainstream use... sure they're charging $20k for it, but that doesn't make it "good".