I suspect it's the temptation of oversubscription. If service A and service B each use 50% of a server, it's so tempting to put them both on one server to maximize efficiency. Even if sometimes you need 4 servers running A and B to serve the load that can be managed with one server each of A and B.

Or if you've broken things up into small pieces that aren't big enough to use a whole server, that can feel inefficient as well.