Because if you allow bxn, your kingside pawns get wrecked. The harder question is why not nd2 or ne5, which also avoid this. Nd2 seems like it may run into some dfile congestion issues / problems defending our dpawn, after rfd8. Ne5 leads to a pretty boring position after nxn. I guess the idea with be2 is that it clears the dfile so we can shift the e rook, which isn't useful on e1, to d1. I know you are saying "but its on a half open file!" but in the endgame rooks are often best behind your pawns, especially your pawns that are vulnerable or can become past pawns. And there was no penetration squares on the efile for the rook on the half open file -- you aren't restraining e5, because black probably isn't playing that anyway, since after e5, d5 then creates a passer.
Anyway, that is how I see it with the benefit of computer assistance. Not allowing bxn is partial credit & realizing that the e rook will be better behind the dpawn is a significantly more advanced point.
Looks like you are about my rating so take it with a grain of salt.
Hello, in this position engine suggests Be2 as one of best moves for White, does anyone have a clue why?