It actually does have to do with a lack of nuance. To say that the brains of those during adolescence up into young adulthood are likely not fully developed is correct. To say that this impacts their ability to make decisions under certain circumstances is also correct. The cognitive control system is involved in more careful and deliberate decision-making while the socioemotional system is involved in more intuition-based decision-making. The socioemotional system begins its development earlier compared to the cognitive control system, hence why teens tend to make more impulsive decisions and engage in more risky behaviours.
The brain system involved more in reward-based decision-making is more active during this period compared to the area involved in planning (the cognitive control region). This isn't something that can just be simplified down to just young people having less experience, even if that may play a small factor in this. This particular pattern of behaviours that we see amongst adolescents are patterns of behaviour we even see in other animal species during their periods of adolescence as a result of the differences in the timing between the development of these two brain systems.
While I understand that you agree with my overall point, I just feel the need to point out that the whole "the brain is always developing" argument just doesn't work here since we are specifically focusing on a major developmental milestone that does have a noticeable impact on how we make decisions. The issue is with people oversimplifying things. From a neurological perspective, they are sort of right, as in that the maturation of the prefrontal cortex does finish, on average, around the age of 25. What they get wrong is that this is an average and not a magical number (just like you said), that some studies now suggest that the process of myelination around this region can continue up into your early 30s and that younger people are still capable of good decision-making (on par with that of adults) under the right circumstances.
A lot of this does come down to a lack of nuance since most people usually hear about the "25 means good big brain, ooga booga" factoid and just roll with it because it is science. They don't understand the full scope of how complex this topic really is. People generally tend to like having clear-cut ways to categorize things, so they default to treating 25 as this sort of magical number, not realizing that it's not. To add to this, most people don't have a background in neuroscience which might make understanding the complexities and nuances of this discussion more difficult. Thus, I don't think that we can just make this out to be a case of people "people justifying their opinions with arguments they don't care about". In my experience, in most cases of having heard this argument being used, it was usually in progressive spaces from people who actually do seem to mean well and who genuinely believe in this fact wholeheartedly, trying to use it to explain certain experiences or feelings in their life.