As a fellow not-fan of YA, here are my two cents:

1. Simply, because of the target demographic. The major consumers of YA tend to be teenage girls who are either experiencing their first romance or are curious about it. YA novels provide an outlet for this, and there’s nothing wrong with that when done well. When it is not done well is another story, but again, nothing wrong with it in principal. Teenagers are going through puberty, they are curious, and literature is a perfectly acceptable place to go looking for answers (better a YA book than a porno).

2. If by “staked” you mean the characters are inseparable and their personalities disintegrate, then yes, it makes the relationship and characters unlikeable. IMO, a healthy relationship is one in which two (or more) people come together because of mutual interests and chemistry, but do not lose their identity in the process. An example of the inverse of this would be something like the relationship in “The Owl House”; the relationship between two of the characters is insufferable to me because it seems that they are only defined by that anymore now. A lot more to say about this, but I am not trying to write a master’s thesis.

3. To be fair, this is an issue of bad editing rather than the YA genre. Lots of authors write themselves into a corner, say “oh shit” once they realize it, then have to deus ex machina their way to a satisfying conclusion. This often leads to rather abrupt and anticlimactic endings which, as you perfectly said, makes you wonder if everything leading to it was worth it. Another example worth giving is the final season of “She Ra: The Prinesses of Power”, which is a show I like very much, but completely flopped in the last season for this very reason. The actual conflict of the season felt rushed, unexplained, then suddenly there was a big battle and the day was saved; that’s not an issue of the show being targeted towards younger folks, that’s an issue of bad writing.

With regards to your last question, my main concern with YA is that I am too old for it. I loved “The Hunger Games” when I was a teenager, but I am a teenager no longer. I rarely find relationships in YA books healthy, there is either too much or too little worldbuilding, and the stories become ridiculously convoulated when the first book inevitably turns into a multi book series where each new entry is longer than the previous one and it seems like it’s going nowhere and suddenly it just ends.