The most successful project (internet + web) did a much better job.
But legacy wise, we have a breakdown of boundaries at so many levels of the stack.
Breakdown of boundaries meant squeezing more performance from both memory and CPU which were expensive and lacking.
Whereas Apple can release a new device on new OS with some custom hardware/drivers and as long as the internet + web parts are compatible with those interfaces there is no issue for customers.
Maybe you're right though, but also isn't that what Posix was meant for?
Also for things like Wi-Fi, they define those standards but the underlying vendor implementation is totally custom and can get really ugly still, just like an OS.
Even "relatively" simple monolithic systems are nightmarishly complex to change.
Apple doesn't really give a shit about backwards compatibility, past the bare minimum to keep their platform devs from revolting.
POSIX, originally, was intended to provide a solution to every-vendor's-Unix problem. A nice side effect of it was absolutely that we got (mostly) standard interfaces.