What other people think of you is also an enticing reward. Getting to platinum rank by cheating has advantages.

Additionally, you earn cosmetic items, and you can earn them at a faster rate with less effort by winning consistently.

Finally, some people simply enjoy winning, and only turn their cheats on for a "comeback."

An additional issue is people running bots that passively and extremely poorly play the game, but just active enough to not sound any alarms. They grind out a bunch of ranks by losing constantly, get some good equipment built up, and then sell them.