It's the safest default. If you want a filesystem that automatically downgrades the safety of your data when things go wrong, you should have to opt-in to such behavior (and btrfs does make that possible).

There's less risk of damage from running off a single disk in a faulted mirror than to rebuild the mirror. The stress the drive is under to rebuild can and will kill drives at the end of their life.

If one half of a mirrored pair dies, you're left with one copy of the data on a drive that's at risk of also dying, plus whatever portion of the data you have backed up. If you then write new data to that drive, it isn't mirrored anywhere and is also guaranteed to not be backed up yet. So the newly written data is at equal or higher risk than the data that was already on the surviving drive. It's not hard to see how silently accepting new writes that cannot be mirrored could be an unacceptable risk to some end users.

With btrfs, you can easily add a new drive so that new writes can be accepted and mirrored immediately, then start rebuilding the old data with lost redundancy (possibly after running another incremental backup, which will be less stressful to the drive than the full rebuild). But the "add a new drive" step happens outside the kernel, in userspace and possibly in meatspace, so it can't be a default action for the kernel to take.

If that is a concern, then it sounds like running mirrored disks isn't sufficient for your use case and maybe require three disks to enable a disk failure and a rebuild to not cause an issue.

Also, if you're happy running off a single disk in a faulted mirror, then I'd question why you've got the mirror setup at all.