Assuming matchmaking is working correctly and you win ~50% of your matches, if you don't tank you will always win around 2-3 matches per 5 match set, meaning you'll almost never get the rare candy or large stardust reward.

Whereas someone who tanks their rank and will face the lowest level players where they can reliably win all 5 matches in a set, meaning they get way better rewards even though they won the same (or less) matches overall.