Let's not pretend that hosting an OSS chat server is a walk in the park for every enterprise just because you personally got one up and running in a weekend.
You'll find a lot of times the champion of the new hotness moves on and leaves someone else to maintain the old hotness at costs that were never factored into the original deployment.
As I noted in another comment, it's a trilemma where you have to pick and choose your battles. The initial deployment time is rarely the most important factor when introducing a new stack within an organization.
For example, large MMO guilds have a quite different regulatory environment from, say, a large healthcare organization in the US governed by HIPAA. It's an extreme example, sure, but a homegrown, dumbed down solution, might be the only way to ensure regulatory compliance.
It doesn't take a lot of imagination to come up with other scenarios. I can't defend Uber doing it because I'm not involved, but large MMO guilds are not archetypal of enterprise systems.