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I'm getting the feeling there's a bit of dunning-kruger going on here, and that it was unfair of me to be quite so glib about this in my initial comment. A short overview of why it's Not That Simple:

There's the huge issue of both 'volunteer personal liability' and 'convention liability for enforcement.' You can't have your volunteers handling 250-500-1000 people's IDs (one hazbin panel was in a 2000 seat room and still overflowed whoops), and the con really doesn't want to take on the liability that would come with enforcing ID checks. It's why cons with beer gardens have outside (usually facility-provided) staff to manage them, they don't just use their own crew, you actually need training to deal with that. And then, if the con is enforcing age requirements they open themselves up to be sued for failing to enforce it (and causing 'emotional distress' to a panel viewer that was uncomfortable, this is a real example I had to deal with, it was even more stupid than it sounds) or lawsuits for unfair discrimination when someone who looks 14, but is in fact 20, is denied entry by a frazzled and overworked volunteer.

And then logistically: you don't want your volunteers fielding all the complaints at the door, people won't be able to get in or you'll run out of volunteers. This isn't a concert venue or a club, these things are huge and are the most complex crowd management scenarios that exist after Disney World. There's no expansion of a line like you get with security checks outside of panel rooms because convention centers are designed to make movement of people from spaces as efficient as possible. Introducing artificial bottlenecks into spaces like that will not only impede foot traffic and violate fire codes, it can be actively dangerous. Introducing excessive foot traffic for avenues not designed to handle peak loads like that sets up a perfect crush situation, and it absolutely has to be discussed with the venue beforehand to see if it's even legal.

These aren't insurmountable problems, but they are large problems that come on top of a million other ones. A convention's resources are much better spent managing problems, not creating new (and potentially very severe) ones for themselves.