Why is this?

If you've truly watched that video then it must be a long time ago and are remembering it wrong. Because it does say exactly what you're saying early on in the video, explaining the studies that show how people are now younger from a medical point of view. You then clearly see that the age difference reported in the study from a medical point if view is not nearly wide enough to explain the magnitude of the difference of we perceive in real life.

This is why video then shifts away from the purely medical perspective towards the more subjective reasons that could affect how we perceive people's age. Of course it's not gonna be backed by medical research to support this because the other reasons for this phenomena has absolutely nothing to do with medical science. Medical science doesn't give a shit about the evolution of fashion in haircuts, makeup and clothing. But that doesn't mean that it cannot have an effect on people's perceptions of other's age. It is obvious in the examples provided in the video that this has a far greater effect on the perception of someone's age than the medical explanation alone.

The meme itself is obviously about people's perception of people's age, which is affected by both medical and subjective factors like the evolution of fashion. Trying to pretend that only the medical factor counts is, essentially, ignoring the other half of the argument just to make yourself sound right.