Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Signing in isn't necessarily required every month, but it can be required 'evey now and then' and that's actually common for online accounts. For example, I have to sign in to Microsoft Q&A every now and again.
There can be several reasons why this is needed. Some accounts may make you sign in from time to time to prove your identity, as a way to stop someone else from using your account. Another reason is that the website owner - in your case, Microsoft - is reconfiguring its servers so everyone (or evryone within a certain range of IP addresses) has to sign in again. Yet another reason is that your connection uses (a tiny amount of) the server's resources and if the server thinks you're no longer interacting with the website you may get signed out so as to return those resources for use by someone else.
None of these are signs of a problem. There's isn't anything that needs to be 'fixed'. Just part of living in a digital world.
With that said, it is possible to remain signed in for a very long time, but not permanently, if the website offers that. For example, Google offers that, so I am almost never required to sign in again. They do this by recording data akin to a cookie on your hard drive, which keeps your session alive. Then again, I use my Google apps every day so there's no reason for Google to sign me out in the first place.