Airlines don't want to have to commit to always using the same model of plane. It's pretty common they downsize the plane if not many tickets have sold or upsize if they're selling well. They also do swaps in case of mechanical failure or large delays.

It's the reason they won't let you get a seat reservation till near the flight date (since different planes might have differing seat layouts)

Taking away this flexibility could easily wipe out their entire margins.