Cynical answer: Because they've found with GTA V and other games that this works quite well to get people to buy it multiple times. First for the current console, then for PC, then for next-gen console if you get lucky.

Also while they are technically similar, it's still separate optimizing/QA work, so staggering release makes sense vs waiting until the work is done for all platforms. And maybe it makes the rollout for the online part easier if the easier-to-analyze/-hack PC versions come later.