I imagine they would need quite a few servers to replace their current setup.

Then there's also the overhead of setting up and maintaining the hardware in their location. It's not just a "solve this problem for ~$2,000 and be done with it".

I don't know the actual specs or requirements. Maybe 1 build server is sufficient, but from what I know there's nearly 4,000 apps on FDroid. 1 server might be swamped handling that much overhead in a timely manner.

One server with today's tech can easily replace several servers that are 12+ years old. 4000 apps doesn't sound like a lot of work for one machine, unless you assume almost all of them are releasing new builds more than once a week. A 16-core CPU can rebuild a full Gentoo desktop OS multiple times a week.