... keen to say that those who don't accompany their children are not "supportive" or "do not care".
Have not knowingly seen any such criticism, so will have to take your word for it.
"to treat their offspring as being mature enough to make this important decision for themselves."
That's all well and good - lots of posters have shown they took the decision for themself, whether or not their parents also went to an open day or several open days. Being able to discuss is part of that decision making, so long as views of parents are not taken as "law". A second opinion, having someone else point out something missed by the young adult, is probably desirable. Good for you that you went off and got into Cambridge, self-confidence as well as academic ability seem to be yours by the shed load, but not everyone is the same, and if you cannot accept that I question you being a fully 'rounded' adult.
"for sure, a 17 year old seems young from a 40-60 year old's perspective, but so does a 24 year old. Surely no-one here would accompany their 24 year old to an important job interview."
Not the same by a long chalk. No, I doubt anyone would expect to accompany them, but there might well be a wish to discuss the pros and cons. Have just spotted some Google advertising for 4 jobs in IT with MI5. Clearly states that one is restricted in who one talks to - limited to close family and partner only.
I would expect any 24yo to make the final decision themself, and probably not to be too swayed by other input, but if that "important" job interview was for a position overseas, or in a security service like MI5, then a bit of discussion with an elder would seem beneficial. You couldn't chat with your mates in the pub about it, after all!
"A healthy adolescent enjoys that independence and a truly caring parent encourages that."
Part of that caring includes taking an interest and being as involved (or not) as the offspring wants. You've surely seen instances where people have posted, saddened that their parents didn't seem to show that care, at the time, though there may well be reasons that were not be explained at the time (pressure at work, health concerns, whatever). You seemed earlier to compare parental involvement with interference, now you push your view that a caring parent let's them get on with it, even if someone is immature and could make some bad decisions. Others feel different - there should be room for all views across the spectrum as this isn't a binary choice.
"if 1 in 10 teenagers viewed with their parents, it would probably be fine. But there is something wrong with a society where the proportion is so high."
In YOUR opinion. Others clearly feel different. Hope you can accept they're actually NOT WRONG and that your view might be shared by around only 50%, and not 90% as you appear to want.
It's a massively and constantly changing society. Technology has changed, attitudes have changed, but many parents (for whatever reasons) have decided to make time to accompany, mostly with agreement, I assume, and hopefully not too many are there for 'control freak' reasons to micromanage what happens.