Who says that they don't? Microsoft has heavily invested in advertising since Windows 10.
Keep in mind that Twitter does the exact same thing (locking accounts and requiring phone numbers to unlock) since a while ago and recently got caught "accidentally" using those numbers for tracking reasons. Facebook also got caught doing the same some time ago.
They certainly aren't using it to verify I am who I am....
I had them lock a MS account I made for a 2nd/alt account (Something I bought years ago to help when I made plugins and game content)..
Account got immediately locked the second I tried to use the launcher for "suspicious activity, give mobile number NOW to regain access".
I tried fighting it for a month, Mojang buried their head in the sand. I've even seen youtubers with multiple millions of subscribers get hit by this bullshit, with no help from Mojang.
They already have the email address I associated with the Minecraft account to begin, that should be good enough...