Because microservices have a granularity that allows a sort of distinction as an architecture that a big ball of mud cannot provide. The sign that the design is bad in the first case is that the services are far too chatty, but that is not a bright line distinction: it is always subjective if the services are chatting too much or not, when is the messaging some kind of messaging spaghetti? The mere fact that you developed your monolith into a big ball of mud is bad design manifest. So microservices make it harder to identify bad design. Designing a modular monolith from the ground up will feel like overengineering to many, until you arrive at the big ball of mud and it is too late.

Simplistic is often sadly seen as an effective replacement for the difficult achievement of simple.