The answer is part scientific, part administrative, and part practical.

Scientifically, the spike protein is made up of many different epitopes (smaller parts of the protein that are recognized by antibodies or T cells). Some of those epitopes still convey protection for current variants.

Administratively, Covid vaccines still have to go through hoops that eg influenza vaccines currently don't, so it's easier to just use what's already gone through trials and approval processes. Soonish, the vaccines can bypass those regs and update as fast as flu vaccines do. Whether that is helpful or not is up for debate, as we've seen that flu vaccines are often outdated by the time they're released.

Practically, if the vaccines are still effective, there's not a lot of pressure on eg Moderna or Pfizer to "retool" the production lines to make an updated vaccine.