I mean, if that's how you choose to use preseason - just playing starters the entire game for XP reasons - I suppose that's fine, you play the game however you want. I personally make it a point to rotate my backups in to see if they offer something for my team on the field for the sixteen games later that count.
Should the game have mechanisms and presentation components to emphasize the whittling down of the roster on its own? Sure. Should every team's CFM roster also have a practice squad? Absolutely. That said, there is nothing stopping you, the user and the head coach, from subbing in players further down the depth chart on your own to see how they do in a game situation which won't affect your chances of making the playoffs. Heck, the game already does the mass substitutions automatically, so there's a starting point. That's why the game starts the preseason with a 75-man roster limit per-team and gradually reduces it to 55 (yeah, it's supposed to be 53 but Madden is Madden), to encourage you the user to evaluate your players on your own to determine who to cut after each game.
It requires a little more effort on the part of the user than desired, yes, but the ability to evaluate one's roster is already there.