Heh, this is interesting, hardcore JYP stans vs JYP stans is NOT what I'd expected to see, this is a rare occasion, I'm all for OP being focused on JYP groups instead of other groups like usual
As for the question, I think it's a simple matter of the coaching program and selection focus:
- when JYP, YG and Boa were all jury on Kpop Star, both JYP and YG had given up on certain candidates, saying that those candidates had hit their roof and that they didn't see how the candidates could improve. Boa said 'I know what's the issue, give them to me for training', and the next assessment those same candidates that JYP and YG had given up on, absolutely slayed in their performances vocally, leaving JYP and YG with mouths wide open.
- when Minzy in 1 of those variety shows got a vocal training session from 1 of the SM coaches, she improved so considerably, finally leaving some singing flaws that had stuck with her for ages behind and elevating her singing that Minzy almost burst into tears and said 'now I can finally grow and improve'
There have been quite some examples that show that JYP's and YG's training and coaching programs has some blind spots or flaws when it comes to singing: either the trainees get to learn wrong techniques to produce a distinctive sound (YG) or breathy sound (JYP), or the coaching programs lack some basics when they're taught singing, leaving it up to a trainee's basic individual skill when it comes to hitting and holding the right notes, singing stable and harmonization.
Same for selection of trainees: seems like SM's selection program has a focus to at the least have 1-2 vocal powerhouses in each of their groups, aside from having all the other members improve their overall singing skills.
With YG and JYP, both their selection program as their training program just lack that attention to vocal excellence. It has just not been a priority to them.
Although, I understand that with YG's next girl group, they have claimed that it'll have vocal powerhouses.