This is only because we arbitrarily picked an age - we used to consider people responsible for their own actions at much younger ages, it makes sense to change the number when we get new science supporting that. If the argument is that we shouldn't use brain development as a reason to restrict young people, then why not lower the drinking age again or get rid of it?
You can join the military and drive a tank into war at 18, but god forbid we let you have a beer afterward.
It seems like you're arguing that people shouldn't fully participate in society until they start losing their ability to learn. Have you met people who've lived overly sheltered lives until the age of 25? Their tolerance towards risk is low. Their ability to adapt to new experiences is busted. Forcing that on a society wide scale would be nuts.
The science does throw in a lot of complexity between the social ideas of freedom and the physical effects of alcohol. But have you met 18 year olds? They're drinking.