Because my 99's with 99 everything are molasses, regardless of their weight.
I ran into that this weekend.
I played a few Champion games and someone had HB Jonathon Brooks. My 99 speed and 99 Accel players could not keep up with him or tackle him when they did have a good angle, which wasn't often.
It is total BS, but common knowledge how older players get "nerfed" and there are certain "glitch" advantages to constantly upgrading your team with the newest players, especially LTDs. If I play a team that has an LTD, I will have players that uncharacteristically take bad angles, fumble the ball, do not block, get in the way of the runner, throw bad passes, drop passes, start diving for passes and drop them, lag behind in coverage, forget they are in the middle of covering a receiver, let the ball bounce off their body rather than intercept a pass, run slower than normal, seem to have no acceleration, etc.
Yep, so many times it don't make sense. My only thought is the abilities and stamina play into it.
But even before abilities became total takeover of this game i noticed it. I'd get a card and be super impressed. Then 2 weeks later it's like the cards speed and acceleration ran out of gas. Probably EAs way of getting you to upgrade and buy packs.