It's arbitrary but probably based on test data of user experience when load testing higher limits. It's exactly the same reason behind the "arbitrary" limit of

It's arbitrary but probably based on test data of user experience when load testing higher limits. It's exactly the same reason behind the "arbitrary" limit of 10 users being allowed to access to a shared mailbox. It's something that makes sense 99% of the time, but for that 1% of users it's pretty painful to deal with.