There's a lot of reasons why. A lot of these dates are chosen months and months in advance with marketing and release strategies planned for those time periods as well as physical manufacturing being accounted for plus dev time cycles are pushed against these proposed dates. It's also not like everyone in the game industry is getting together talking about their projects and release schedules and strategies openly to make sure the calendar year is properly saturated.
What I'm saying is if someone were to push a game a few months out for another companies game it not only could potentially harm the budget as well as any projected revenue from the game thus inflating the needed recoup and possibly harming the game Dev's you also harm the face of the company. You want to believe in your product both in house and in public. Katowing to another game because it's bigger than you only hurts your image and it does. I've seen projected numbers before and after a delay against six month sell throughs after a delay. Not only is it a knock against the marketing strategy and budget it harms public image.
It's not very simple.
