Because it's a complex infinitesimal web of code and changing things sometimes have knock on, unexpected or unintentional consequences. You only need to look at any live service game. I remember Apex's hit detection went slightly off one update, but it was unnoticed until out in the wild because its impossible to stress test every little thing and the difference was so hit and miss that it had to be out there and getting played for players to notice something was a little off. Further inspection confirmed it and it got fixed. That's one example in thousands. If you combed and play tested a patch so utterly comprehensively that nothing got through, we would never get patches and the manpower would be absurd. As a gamer you just accept there may be a few teething troubles sometimes and get on with just enjoying the game, if it ain't totally broke.