Many, many people do not have the capacity to build and maintain custom solutions (whether in-house skills, or simply bandwidth) and therefore outsource to vendors.

It's an incredibly common aspect of business. Enterprise level contracts often include the sort of white glove service to help fill in these sort of gaps. On simpler plans, having the tooling provided frees up just enough capacity to handle the exceptions to keep the process running smoothly (since one doesn't have to build and run simultaneously).

Sometimes people want to minimize the hassle with stuff. It's why car washes and oil change places and coffee shops exist.

For the last 15 years, I'm that person in the company who always said "let's not build it ourselves".

In the last 6 months, I'm that person in the company who always said "let's just build it in a few days".