Because it means BYU is actually good again and relevant and other people are jealous. You never hear this argument when BYU sucks.

If missions were such a huge advantage athletically, just about every coach would be encouraging their players to take 2 years off and serve a mission, join the Peace Corps or serve in the military. Obviously nobody's doing that because they know it disrupts and hinders player development and team continuity.

Even BYU coaches know if anything it's a huge disadvantage for them. The only benefits for players are spiritual for the most part.

Heck, even the great Lavell Edwards discouraged his players from serving missions initially. (He later had a change of heart.)

Also as many others have pointed out, the vast majority of our star players in both football and basketball did not serve missions. That's not a coincidence.