Because the rigour and quality of work that is needed to do good software to get to market is insanely higher than the one required for keeping the money machine alive once you are a monopoly. So as you hire more and more to get your hands in as many pies as possible, have fewer and fewer experienced staff to train the new hires, and do questionable hires in leadership positions, the culture invariably shifts to doing less and less effort, or putting effort in the wrong place, and there you have it.
Now in Microsoft's case, the quality was never that high to begin with (but the scummy practices made up for that), and the pockets are deep aplenty, so I think we are in for quite the shitshow (it can, and it will, get even worse).