Because a DVCS tool like Git makes commits much less costly than older tools such as CVS or SVN. The dynamics (both social & personal) for commits are different.
My guess is that you understand Git commands but you're using the SVN/CVS mental model of treating commits as "sacred" markers. If someone commits in those older centralized systems, they could potentially break the build and stop the team's productivity. This leads to strange social dynamics such as programmers "hoarding" their accumulation of code changes over days/weeks and then they later end up in "merge hell".
Because Git "commits" have a private-then-public phase, the programmer does not have to be burdened with affecting others' productivity with their (sometimes spurious) commits. They can have twitchy trigger fingers with repeated "git commit". The git commits can be treated as a personal redundant backup of Ctrl+S (or vi ":w"). (Or as others stated, the git commits and private history become an extension of their text editors.) They don't have to hoard their code changes. Because of the different dynamics, they don't necessarily have an automated Continuous-Integration complete rebuild of the entire project triggered with every commit. To outsiders however, many of these commits are just "noise" and don't rise to the same semantic importance that we associated with CVS/SVN type of commits.
In this sense, "rebase rewriting private history" does not mean faking accounting numbers like "Enron fraud" and throwing auditors off track, but instead, it's more like "hit backspace key or Ctrl+Z and type the intended characters."
In CVS/SVN, the "commits" are a Really Big Deal.
However, in Git, the "commits" are Not a big deal and closer in concept to a redundant "Ctrl+S". It shifts the Really Big Deal action to the act of "applying changes or merges" (e.g. "patches" is how Linus Torvald's often describes it.)
