Once a WR has a first down, gaining a few extra yards between the 20s is next to useless, and fighting for them is a losing proposition. Yards are cheap in the NFL. Possession is expensive. When WRs fumble, the defense recovers more often than not, so there’s really no reason for them to fight for a yard or two while risking taking a hit and possibly fumbling. Get to the ground or out of bounds, and live to fight another day.