DVCS have a DAG of commits, which can get arbitrarily complicated and difficult to reason about.
Rebase-based workflow results in a linear history of commits, which is easy to reason about.
Merge-based workflow results in an arbitrarily complicated DAG of commits which is difficult to reason about.
That's all there is to it.
The repo rapidly turns into spaghetti with a merge-based workflow because all the work in progress is now part of the historical record for any particular commit to main. You're not looking at a single commit, you're looking at a chain of commits and a merge node. Now imagine there are 20 people committing to the same repo. Your branch factor is exploding! Imagine the picture of my merge-based workflow, but multiplied 7. It rapidly becomes very difficult to navigate.
The DAG is important as a historical record because you have to go back to it on a very regular basis.
It doesn't change the workflow from a "do your work and commit to the repo, periodically sync with main" perspective. It changes the workflow from a pull request perspective.
I've found rebase is (slightly) inferior for conflict resolution and reintegration--mainly because unless you squash your commits down to a single commit, you may need to resolve the same conflict as many times as you have commits after the conflict in the worst case, which is irritating. Merge is just one and done. But that's a minor thing.
Having used SVN (briefly) and Mercurial (a lot) and Git (a lot), SVN pushes you into dealing with a single linear history of commits, with cross-branch merges being extremely painful and error-prone if anyone else has worked in the same area. DVCS like Mercurial and Git allow you to do whatever you like for the history, and cross-branch merges are generally easy and pain-free. I can't say anything about the underlying implementations and why it is that way, but that is my lived experience.
Most of the time with Mercurial and Git I can let the merge tool resolve differences if there are conflicts with the odd line needing manual intervention. Most of the time there aren't conflicts.
And having used SVN a bit...yeah, it's the stone age in comparison. Sometimes you have to make that tradeoff because you can't store everything locally, but I haven't enjoyed the times I've had to use SVN after having used Mercurial and Git.
And Git's user interface sucks.
I need a Mercurial skin on top of Git so the commands make sense.
merge-based ... rapidly turns into spaghetti with even just a few people working in the repo
Isn’t it just a detail of how graph/report tools work? Can’t they track these merge points and “rebase in their ram”? I don’t get how a graphical representation of merge points may change the workflow.
One more thing that is unclear is why some people think that rebase is somehow superior in terms if conflict and reintegration. Like they “had issues with svn and now that rebase is a thing, issues gone”. Maybe they didn’t understand that you have to sync-merge your branches (effectively rebasing) periodically to not diverge from trunk (or parent branch) too much?
Added: I know rebase is not congruent with what I’m asking, but my questions are more about how git folks think, not about how git works. Cause I often see its comparison to other VCSs and claims that are vaguely or simply untrue about git competitors. As if before git there was some stoneage.