For some teams it's just a philosophical decision, but many larger enterprises have mandatory requirements which makes OSS software much easier to adopt (this is in relation to infrastructure products of course).
You mean that enterprises would prefer on-prem, right?
But if I'm an engineer working on a large enterprise, what's the difference between a SaaS providing an on-prem option, and an OSS? It's not like I can just git clone a repo and run it on my large enterprise cloud environment.