Because you could keep him a year or two and then trade him for more than 2 first round picks? Or you could trade him to a team that doesn't get one of the good quarterbacks in the draft.
Deshaun Watson, a worse quarterback, got more than 3 first round picks.
Also, if the Jets thought Lamar was a good option, why do so much to get AR? It would have been malpractice to not have SOME idea what the Packers wanted before going to California to meet with AR.
No one knows how long Lamar will go before signing with another team or back with Baltimore anyways.