In terms of ratings, you have to take the with a grain of salt. You can put vilma at DE and get like a 99, but in terms of practicality while playing he is worthless.
I think in regards to how they actually play, awareness, play recognition and zone/man coverage are the reason why DE's can't play well as LBs.
LB's with low awareness/play recog are basically unaffected on the line. DE's playing in space however, look terrible with the same rating.
There are a couple of ways they could fix it I suppose.. add new positions (3-4 rush backer, 3-4 DE etc.) or tweak the way acceleration and speed work at different positions.