Most of the changes aren’t relevant to upstream - a lot these companies seem to be using Postgres as a frontend over some kind of secret sauce, usually related to custom storage or deployments or the like. Postgres seems very modular and pluggable that way.

I assume for the common Postgres core changes everyone is already incentivised to contribute changes back upstream - the companies are banking on Postgres popularity can need it to have an excellent reputation even outside of their specific customised offering.