The tendency for adults to be portrayed as flat stupid (in a lot of pop culture aimed at the youthful market, not simply YA books which I have minimal knowledge of) is kind of mind-boggling in a pandering way, or as you say, the adults just don't exist.
Only because I've done a bunch of research into the market (and keep in mind, I'm doing MG, not YA... so slightly different), this is a pretty important trope for a reason and goes as far back as the trombone-voiced invisible adults in Charlie Brown.
Think of it like technology. In any modern day horror or adventure story in order to achieve tension, the first thing the writers do is eliminate the technology... any ability that the characters have to access the outside world. So in a horror this means they will lose their cell phones or the computer system will go down, or it will be the technology itself that is the problem. Or, in large scale horror like The Walking Dead all technology will be eliminated completely. This gives the characters that sense of isolation, that inability to access help easily, the inability to band together in big numbers, etc... basically it creates more challenges = more conflict = more tension = more story. The same goes for outdoor adventure stories, wilderness survival stories, you get the idea.
With kids, adults are the like the equivalent of technology. If kids can just turn to adults all the time, then where are the challenges? What is the conflict... "Mrs. Watson, Joey is teasing me." "Joey, go the principles office, you are suspended." THE END.
Basic analysis but you get the idea. When a MG or YA story is filled with smart, competent adults then it takes the onus off of the young characters to do their own problem solving. They become passive instead of active, simpling asking for help from older, wiser adults all the time. When you give your story the Charlie Brown effect (eliminating the adults by making them either clueless, absent or downright invisible) then the story is focussed on the active participation of the young characters. Their choices, their mistakes, their obstacles, etc.
Does that make sense?
Many MG or YA books have amazing adult role models, but you will see that once the climax comes, just like in an adult horror or adventure story, something will happen that will eliminate that resource (like Dumbledoor's death) so that the young character is forced to act on their own. To take what they have learned and apply it independently without the help of their mentor.