It takes X*N amount of work to merge change X across N repos. 1 repo just takes X.
Then there's version management. Do all your repos use the same versioning scheme? "They should", but in the real world, they sometimes don't. Whereas if you only have 1 repo, you are guaranteed 1 versioning scheme, and 1 version for everything.
How do you know which version of what correlates to what else? With N repos, do you maintain a DAG which maps every version of every repo to every other repo, so when you revert a change from 1 repo, you can go back in history and revert all the other repos to their versions from the same time? Most people do not, so reverting a change leads to regressions. With a multirepo, there only is one version of everything and everything is in lock-step with everything else, so you can either revert a single change, or do an entire rollback of everything, with 1 command.
How do you deploy changes? If each repo has an independent deployment process (if your repos even have a deployment process that isn't just waiting for Phil to do something from his laptop), are you going to deploy each one at a time, or all at once? What if one of them fails? How do you find out when they've all passed and deployed successfully? Pull up 5 different CI results in your browser every couple hours, and when one fails, go ask that team to fix something? If you only have 1 repo, there is 1 deploy process (though different jobs) and merging triggers exactly what needs to happen in exactly the right order.
The reason people use multirepos is they don't want to build a fully automated CI/CD pipeline. They don't want to add tests and quality gates, they don't want to set up a deployment system that can handle all the code. They just want to keep their own snowflake repo of code and deal with everything via manual toil. But at scale (not "Google scale", but just "We have 6 different teams working on one product" scale) it becomes incredibly wasteful to have all these divergent processes and not enough automation. Multirepo wastes time, adds complexity, and introduces errors.
Then there's version management. Do all your repos use the same versioning scheme? "They should", but in the real world, they sometimes don't. Whereas if you only have 1 repo, you are guaranteed 1 versioning scheme, and 1 version for everything.
How do you know which version of what correlates to what else? With N repos, do you maintain a DAG which maps every version of every repo to every other repo, so when you revert a change from 1 repo, you can go back in history and revert all the other repos to their versions from the same time? Most people do not, so reverting a change leads to regressions. With a multirepo, there only is one version of everything and everything is in lock-step with everything else, so you can either revert a single change, or do an entire rollback of everything, with 1 command.
How do you deploy changes? If each repo has an independent deployment process (if your repos even have a deployment process that isn't just waiting for Phil to do something from his laptop), are you going to deploy each one at a time, or all at once? What if one of them fails? How do you find out when they've all passed and deployed successfully? Pull up 5 different CI results in your browser every couple hours, and when one fails, go ask that team to fix something? If you only have 1 repo, there is 1 deploy process (though different jobs) and merging triggers exactly what needs to happen in exactly the right order.
The reason people use multirepos is they don't want to build a fully automated CI/CD pipeline. They don't want to add tests and quality gates, they don't want to set up a deployment system that can handle all the code. They just want to keep their own snowflake repo of code and deal with everything via manual toil. But at scale (not "Google scale", but just "We have 6 different teams working on one product" scale) it becomes incredibly wasteful to have all these divergent processes and not enough automation. Multirepo wastes time, adds complexity, and introduces errors.