They do it for the same reason they do everything so that you are forced to use microsoft products for everything. They love a good monopoly
No, this change does not force you to use Microsoft products for everything. It won't even let me use other microsoft products - I already have Microsoft Edge as my default browser on my phone, which has my stored credentials. In fact, this change forced
me to install another 3rd party e-mail client to continue to follow the workflow, that I have developed over the last year. That product opens my emails in my default web browser - MS Edge, whereas Outlook won't do that anymore.
The annoying thing is that Outlook is perfectly capable of using the default browser. If you reinstall Outlook, it works just fine for a few hours or days until some setting gets synched and the behaviour changes. That means that there is a switch that can
be flipped off just as easily as it gets flipped on if Microsoft permitted it. It says a lot about the decline of Microsoft that nobody has figured out how to tweak that setting - the people with the kind of skills necessary to do that have long abandoned
any Microsoft properties.