The Empty First Class Seat: Why Airlines Rarely Offer Free Upgrades
Despite passengers often seeing empty seats in premium cabins, complimentary upgrades are exceptionally rare. Airlines strategically hold these seats for last-minute, full-fare ticket purchases, which are highly profitable. They also reserve them as a crucial tool for re-accommodating high-status frequent flyers from canceled or delayed flights. Furthermore, offering free upgrades would devalue the exclusivity of the first-class product, discouraging future paying customers. Instead of giving them away, carriers often prefer to offer discounted cash upgrades at the gate to generate ancillary revenue.
Official Source: https://simpleflying.com/why-upgrade-first-class-almost-never-happens-seats-empty/