Babies are cute, look less aggressive, and size misleading. They are easier to sell. I'm seeing 3 and 4 week old hatchlings now. It makes me sick, they are setting newbies up for failure and setting hatchlings up for death.
With the size, I agree. (For this, the reptile store I got Taco from on purpose shows an adult beardie. But I know that reptile stores tend to hide this. Years ago in the US I had land hermit crabs, and I looked up everything online before, so was knowing I need a large tank (I had one that was half of the one I have for my beardie), but still, the chain pet store wanted to sell me a tank of the size of a shoe box.
When I went there to just inquiry about bearded dragons - my plan was: asking if available at all, get enclosure and all supplies, and once that's set up, actually get a dragon - , I was shown immediately their adult beardie, along with what to feed (to make sure I won't have a problem with insects), and that what I would get is a little one taking about a year to reach that size. That's IMHO how it should be done!)
Regarding cute: I doubt that anybody who would find a grown beardie "too aggressive" would find a beardie "cute" - as of then likely not being very much into reptiles. Also (but also sad), some people tend to buy reptiles for an "aggressive" look and might prefer a spiky big one.