It also comes from how git is used. With git small commits are possible. It's nice to be able to manage those and build larger, coherent ones. Also, because of the distributed nature, sometimes rewriting is useful to fix mistakes.

However, yes, that is not the only possible workflow. SVN and CVS have been used on very large projects with moderate success. SQLite development is done in fossil. It's not impossible to do it correctly with immutable history.