The thing is, there's no gaurantee that google nor microsoft nor apple do anything more. With products like nextcloud, at least you can see exactly what's going on.

We already know google, microsoft and apple have unfettered access to your data too. With self hosting, at least I have a chance of knowing when my data's being monitored & I can choose increasingly severe security around that system. The big players can only offer promises with little to no way for us to verify the truth of any one of their statements.

So, unless the big players offer e2ee as a default, it's best to assume the worst from them. With nextcloud it's far less necessary if you're rolling your own (but it supports a form of encryption anyhow)