Agree.

I don’t understand why Microsoft are so bad at this?

They have access to a large percentage of all email traffic to train domain reputation, and spam detection models on yet seem to be notorious for both false positives and false negatives.

Google’s spam filters are far more accurate.

I think they cannot be bothered. Their customers are more tied in than Google's and are not going to switch because a small proportion of emails are rejected.

More specifically they seem to rely IP reputation rather than domain reputation, and IPs do change hands, especially for smaller servers.