Because it's a logical conclusion now with Lamb.
Think about the situation in 2-years:
- Cooper is 29 y.o on a big non-guaranteed contract with only 6m dead money.
- Gallup is up for a new contract and probably a more suitable no.2
- Lamb will be heading into his 3rd year and hopefully demanding the no.1 WR spot.
If Cooper is under-performing his contract cutting him is easy and if he's not it might be that a team offers a trade for Cooper. Clearing 20m from the salary cap, and adding Gallup long-term with a rookie slot WR could easily be the right call.