We don't, and it's hell. I may make a change to X, but some other developer doesn't know it, and then one day when he makes a change to X and tries to pull tho

We don't, and it's hell. I may make a change to X, but some other developer doesn't know it, and then one day when he makes a change to X and tries to pull those changes into Y, now he has to change the bits of Y which are affected by my changes as well as his. I would love to have a monorepo, although I'd advocate for branch-based rather than trunk-based development. Yes, merges can be their own kind of hell, but they impose that cost on the one making breaking changes, rather than everyone else.