When you have a large service, you can end up with a lot of coupling. Meaning that it is hard to have teams that specialize in specific parts of the application

When you have a large service, you can end up with a lot of coupling. Meaning that it is hard to have teams that specialize in specific parts of the application, you need a huge team that knows everything about everything and that doesn't scale as your code and company grows. And you can see the effects of that in reduced productivity and technical debt beyond belief. Even if you hire high-end talent, they will be forced to worship people that have the knowledge about how all the bloated jenga-tower mess works. And that's only the people problem, let's not even discuss how you build, deploy, test things in a reliable way without taking days to produce a working build. That, and the fact that your quality of life will suck, since you won't be in control of such mess. With microservices you have one service that does a limited set of things, that is easy to monitor, maintain, test, deploy and even entirely replace if necessary.