The answer here is not so simple as "always buy". It depends heavily on context.

I think there are some things that do make little sense for anyone to build unless it's super core. These tend to be low level, cheap developer tools that do a couple of things well.

Trying to "buy and then integrate" larger pieces of software is often more work. Lots of people "buy" because they don't understand the problem. It's a mental opting out. This leads to disjointed experiences internally and externally.

But it all depends on your situation.