Usually when people under bet the river it’s because they want to see you call. You can hear the player saying, “Please call! It will only cost you a tiny bit!”, with their small river bets.
Here’s an idea, why not bluff at the pot by throwing in a value bet on the river? You’ll trick your opponents into folding, by making it look like you’re begging for a call. If your value bets are 1/5th of the pot, you’d only need to win 1/4 of the time to show a profit with this play.