Because people are, by and large, mental.
You’ve got a hardcore group of people that will back the manager because we are Manchester United and that’s what we do, and they won’t back down from that. We could have Hitler in charge, and they’d keep the same dogmatic position. That makes up about 10% of our fan base. This group is best ignored.
Then you have about 80% that make up their mind based on the most recent result or series of results. This group wanted him sacked after the 4-0 defeat against Brentford, a lifetime contract after the Carabao cup final, another sacking after the 7-0 against Liverpool, another lifetime contract after finishing 3rd, sacking after the first ten games of this season, patience after the injuries, sacking after Palace, lifetime contract after the Cup Final. They may also make decisions on the manager based on how often he picks the latest in vogue player. Usually a fairly untested youngster from the academy, or a hip overseas signing with little PL experience. This group is also best ignored.
Finally you have the remaining 10% who form a decision on the manager based on a wide sample sizeof results and performances, overall progress trajectory, taking into account mitigating factors. They’ll also factor in other variables such as money spent, players available, alternative managers available, and long term strategic intent. This group is an endangered species and is often hunted and slaughtered by the other two groups.