This is hardly surprising considering two of the people most likely to disagree resigned from the FDA after the last booster approval, due to their disagreement with the process. [0]
The people who resigned were FDA Office of Vaccines Research and Review Director Marion Gruber, Ph.D. and Deputy Director Phillip Krause, M.D.
After resigning, they also published a critique in the Lancet of the policy of boosters for everyone. [1]
[0] https://www.ft.com/content/af8da7d4-43ea-41d6-90ee-f959b3675...
[1] https://www.thelancet.com/journals/lancet/article/PIIS0140-6...
Also those scientists were essentially proved wrong - the booster almost certainly was better for everyone a year ago.
Further boosters is still a bit more complicated but we are doing the best we can.
However this exact issue you are talking about was debated at the public FDA hearing on whether or not to approve the updated boosters. Most, but not all, of the people on the committee believed that it was worth the risk with incomplete information.
It isn’t ethical or possible to run the studies you want given the existing prevalence in the virus, so you do the best you can. It isn’t fast-and-loose to make decisions with imperfect data when all you have is imperfect data.
Also there were even more interesting and nuanced arguments against the updated booster but evidently you didn’t read the FDA meeting notes and see the arguments from the couple people who voted against it. Oh well.