Continuous integration is when the developers merge their code into a shared master branch at least once a day, ideally several times a day.

A mandatory code review before merging is allowed (that takes some time to be processed) leads to fewer, bigger merges. Imagine waiting for reviews of the 10 pull requests of wrote two days ago, the 10 of yesterday and 5 you created so far today. It's too unwieldy. The latency encourages you to start writing fewer bigger pull requests more spaced out in time.

As an aside, it also tends to reduce trust and cooperation within a team : reviewers become the gatekeepers of the work of contributors. That work is explicitly not trusted until it is checked. Not a great social dynamic I think.