The matter is simple:
Being a great athlete is one thing. Being a great football player is another.
Everett has shown no signs of being a great football player, as much as I wish he would. He's the guy who, because of his great measurables, will have a 8 -10 year career, going from team to team, waiting for him to develop into that great football player.
The Rams didn't draft Hopkins in the 4th round (same round as Higbee) to be a 3rd TE. They drafted him to replace Everett. They have rightfully committed to Higbee who is bigger, a better player, a really good blocker, and stays healthy. Everett would have to become the 2nd coming of George Kittle to stay around for the 2021 season.