It's a little more nuanced than this. Gate auctions are a good solution, but can only happen in the last ~15-20 minutes prior to departure, which leads to "too late" scenarios as everyone is already present/committed to travelling.

Oversell is sometimes known with high confidence at checkin-close (1 hour prior to departure) or even before, due to the fact that oversell thresholds can change throughout the booking curve.

Airlines don't have a current solution to rebalance/correct loads prior to the gate auction.