Most college football players never make it in the NFL, yet there are plenty of entertaining games and millions watch them. The NBA G league draws fans to games and people watch on streaming sites, the same is true of minor league baseball. High school football is popular in some states, and most of those kids never play college football.
If you love the sport you can still be entertained by it below the highest level. If all you want to watch is the NFL, that's fine too. A "scrub league" doesn't have to produce players for the NFL in order for some to enjoy it, and it's great for guys who just want to keep playing football. If one or two of them get lucky and make it big, all the better.
Besides, they're playing in the Spring. It's not like they're competing for eyeballs with the NFL and college football. Baseball just doesn't do it for some folks.