Dependencies can be relative paths. Relative path dependencies will break the build on your machine, so you don't get devs pushing broken stuff. CI/CD servers don't get plugged up with billions of repos building because one changed. CI/CD servers don't need to wait for other builds to finish and push artifacts to do their job. Testers don't need to say "I used version 10.2 of x and 11.5 of y", they can have a single version number for the whole stack.
All these things are possible with microservices in different repos. But they require a lot more time to set up and require regular maintianance. If your company isn't large enough to need that, then it makes no sense to move away from a monorepo.
Arguably no company is big enough, Google has all it's code in a single 80 terabyte monorepo. But I can understand why that wouldn't scale well either...