Because then you get easier friendlies, and the friendly cups give much better rewards than rivals. Loads of people are doing it, it's the opposite of pointless, it's arguably the optimal strategy unless you are absolute top player that can cruise against elite opponents and still get your friendly wins.
Basically if you play in top divs then you get matched with top players in the cups, making it harder to get the full rewards. So people just hang around in the lower divs.
The rivals rewards are really bad in comparison to the objectives they put on cups. The 3/7 wins is just a bonus, some players don't even bother with that, they just stick in div7 and play fut champs, they never need to play rivals except when there are objectives running or perhaps to do evos, because they get 1250 quali points from 9(?)+ wins in WL.
EA really need to buff the rivals rewards for higher divs to encourage players to get promoted, and perhaps get rid of quali points as a reward from weekend league, make everyone earn their quali points in rivals. Maybe even reduce the rewards for the cup objectives, or remove the game limits, or make friendly cups completely random opponents so there's no benefit from being in a low rivals div.
edit: essentially all divisions are is a bottom end threshold that prevents bad players getting in to high divs. There's no threshold at the other end that stops good players getting in to low divs, aside from the ones who have promoted high and have to wait for relegation. I play in the lower echelons and there's really no discernable difference in skill between divs6-8, the monsters going 4+ goals up by HT against me are in all those divs. At div5 I then find the easy games start to dry up (because I'm not very good), because the barrier to entry for weak players is there, and I'll get beaten regularly. You've also got the season cutover when you get relegated 2divs, so as seasons progress the top players that have given up grinding rivals drop even further, a div2 player from season1 could be in those lower divs now.