In my experience even getting all the pieces installed in a testing environment is hard. Specially when you need to make sure that they are all wired correctly, with the correct version, with all their dependencies (maybe different kind of databases, queues, caching layers) and on top of that if services are written in different languages, that adds another level of complexity. Eventually when the system grows big enough and the team of people that needs a stable test environment grows big enough, it just becomes too expensive.

Sorry, I meant automated testing. Getting all the piecies running in a test stage isn't hard.