I have no idea the full truth, but I think it's clear that surgical interventions are problematic (negative side effects) and disappointing (a neo-v is not a v) at best. It would not surprise me if the truth is that almost every trans person finds their surgical transition incredibly problematic and almost 100% disappointing.
I can imagine walking in the countryside, and coming to a stream with no bridge. It looks quite wide, but I am fairly confident. I can imagine trying to jump it and slipping as I land and smashing my knee on a rock. After recovery the knee is never quite the same again. It's not too bad, but I can no longer do long walks without pain, and it tends to play up a bit every day.
That decision to try to make the jump would haunt me. Why did I do it? I knew it was risky, I didn't need to cross the stream and even if I did I didn't need to do it there.
Imagine the same but you are sat there thinking "I have no penis and cannot orgasm. Most gay men are looking for a man, not a feminized man, and not a penis-less feminized man, and I can't find a partner". I just cannot comprehend what that must do to someone, day in day out. The bravery of detransitioners is incredible.
But to the point... I believe that humans have massive survival instincts, and part of that is to lie to ourselves that we don't have regrets. I think many trans people are desperate to pretend it's all great on a "fake it til you make it" basis, but deeper than that is something much much darker - a subconscious knowledge that dealing with the reality of your mind's reaction to the mistaken choice you made is just something that cannot be done without a full-blown all-encompassing mental health crisis descending.
The positive claims about the benefits of transition are in part about not wanting to be the odd one out in the community (a failed transition) without knowing that everyone else is thinking the same; partly about not wanting to let other trans people down by undermining the positive messaging; fear of being attacked by other trans people, but I honestly think the biggest thing is the self conscious knowledge that to admit your mistake is to open a can of worms that might never be able to be controlled.