Because we want the best 5 on the field.
For example using like madden ratings, what if:
You have 5 OTs that are rated 74, 77, 79, 85, 86
You have 1 OG that is rated 86
You have 1 OC that is rated 67
Who are you starting and where? Picking the 5 best pure players is easy. But which of those 5 is most likely able to play center? The guy that is 6'5" 325lbs or the guys that are ALL 6'6+ 350lbs+? Now obviously maybe the player isn't as good at Center as he is at OT, but he can still be a big improvement over that OC rated 67, particularly when the guy that would replace him at OT would also be >67 rating...
Also Jalen Rivers is going to get drafted as an OG. Having OT experience is a very good thing for him. Having Center experience wouldn't be a bad thing either. Plus he seems like a very smart kid. It seems like he fits best among current players unless you want to just go with Rodriguez.