Why should they? They have bulk licensing deals with PC OEMs and large organizations. They've already got the big money. J. Random Hacker building a PC isn't even a rounding error to them.
Quite right in fact limited weak copyright protection helps retain marketshare that might be lost otherwise especially when that weak protection in fact costs little real revenue.
Even weirder is that GitHub hosts all of the tools for activating (i.e. cracking) modern versions of Windows and Office as well. Microsoft really doesn't care.