In monoliths, they generally don't.
There's no logical reason why you couldn't pay as much attention to decomposition and API design between the modules of a monolith. You could have the benefit of good design without all the architectural and operational challenges of microservices. Maybe some people succeed at this. But in practice I've never seen it. I've seen people handle the challenges of microservices successfully, and I've never seen a monolith that wasn't an incoherent mess internally.
This is just my experience, one person's observations offered for what they're worth.
In practice, in the context of microservices, I've seen an entire team work together for two weeks to break down a problem coherently, holding off on starting implementation because they knew the design wasn't good enough and it was worth the time to get it right. I've seen people escalate issues with others' designs because they saw a risk and wanted to address it.
In the context of monoliths, I've never seen someone delay implementation so much as a day because they knew the design was half-baked. I rarely see anyone ask for design feedback or design anything as a team until they've screwed something up so badly that it can't be avoided. People sometimes make major design decisions in a split second while coding. What kind of self-respecting senior developer would spend a week getting input on an internal code API before starting to implement? People sometimes aren't even aware that the code they wrote that morning has implications for code that will be written later.
Theoretically this is okay because refactoring is easy in a monolith. Right? ... It is, right?
I'm basically sold on microservices because I know how to get developers to take design seriously when it's a bunch of services talking to each other via REST or grpc, and I don't know how to get them to take the internal design of a monolith seriously.