It's because your defense is playing Nickel or Dime formation for the majority of the time for some odd reason and it puts you @ DE in that situation. In a normal 4-3, it'll put you at MLB but even with a Base Defense of 4-3, the game doesn't let you select any plays from that formation but instead just Nickel & Dime-- that's the real issue.
I like playing the DE spot but they barely give you the chance to select a formation where your player can play his true MLB position.
Once u unlock the full Playbook at 32000 XP & able to select your own plays, you'll be able to choose a 4-3 or 3-4 formation that will put you at MLB