If you can avoid adding an extra service without paying too much penalty, it means not having to acquire an extra skill or hire another devops person or keep yet another service in sync / maintained / etc.

The cost of adding services to an app is so much higher than people give it credit for at organizations of every size, it's shocking to me that more care isn't done to avoid it. I certainly understand at the enterprise level that the value add of a comprehensive system is worth the cost of a few extra employees or vendors, but if you could flatten all the weird services required by all the weird systems that use them in 30,000+ employee enterprises and replace them with one database and one web/application server, you'd probably save enough money to justify having done it.