Also, there are many different strains of flu. Each year, they make assumptions about which strains are more likely to occur, and those strains are what’s in that year’s vaccine. Sometimes they’re wrong, and other strains prevail. Also, the effectiveness of antibodies declines over time, which is why you need booster shots for some diseases. For example, I believe the recommendation is that you get a tetanus shot every 10 years. If you want more information, the CDC has a webpage “Key Facts About Seasonal Flu Vaccine” that explains this stuff.
To bring it back to COVID, this disease is so new that there’s a lot that’s not known about it, including how effective its antibodies are in conferring immunity, at what level, and for how long.