Because in the end, with a guy like Lamb for example, you are just shooting yourself in the foot. He's clearly shown he's a productive, elite level WR. Why do you need him to prove it AGAIN another year? You are just going to cost yourself more money, and more importantly, cap space.
I get if some guy just catches fire in the 2nd half of the season, you may want to hold off on extending that guy. Lamb? That's just bad business right there waiting (again, unless there is some master plan to tear this thing down to the studs at some point).