For me, there are actually quality posters who aren't legendary members but may not be so consistent and as such it could make you a bit confused.
Legendary tend to speak/ post based on their experience and knowledge - that's more meaningful than the average user who is trying to build an account and earn merit from here and there. So it's true that more experienced user who rank highly have more meaningful quality posts than others - but I'm also not generalizing them all to be the same.
The posting quality standards for each merit source are very different. They can sometimes consider posts like mine to be quality because they contain elements of argument, facts, or so on. Some of these others are ordinary posts that are of no use being given merit even if your rank is Legendary.
What you are saying is nothing but the truth, there are some certain things in life that comes with age and experience. There things you will never understand until you get to certain age and level in life. Here in this forum account ranks is what show how long and how dedicated someone has been in this forum. There are many threads that Legendary Members come across and have learnt from and contributed on that new members can't easily access. Most lower ranked members don't usually have the time to go and read those threads to enhance their knowledge only few does dedicate their time to go in search of these threads and go through them. Getting to a legendary rank is no easy thing I must confess, so there is a clear difference in their posts when compared with that of a lower ranked members, no matter how knowledgeable the lower ranked members are. Just like you have rightly sated lower ranked members has many things distracting them and one of it is growing their account to a higher rank.