

Most likely this is the #1 reason. When Passkeys will become more popular, that will be another problem for regular users unless there is an easy account recovery option.
Another possibility could be switching to local account and deleting MS account, but I would imagine that is more rare and most people would just abandon account. Then it can become the same issue with forgotten password though.
Android I think just uses same credentials you use to unlock account, at least I am not aware of any recovery key. And you are prompted for credentials from time to time so it is harder to forget. I use fingerprint as main unlock + pattern and I have to enter pattern roughly once a week I think.
On Windows if you set up Windows Hello (fingerprint or PIN usually), you are not reminded to enter password afterwards so eventually you can forget it. And if you do not know your password and cannot recover account, you will not be able to retrieve BitLocker recovery key. So fix to this problem could be another annoyance to users if it would be implemented as Android does it.