Others have noted other reasons, but I think the biggest reason is that 1st/business class seats are more comfortable than the crappy crowded boarding area. Economy seats, it's the other way around. And as an added bonus, on something like a 787 where the boarding doors are behind business class, boarding first means you're lounging around in the front of the plane being served drinks and addressed by name while behind you the rabble are are being herded like cattle into their tiny seats at the back.
For 3x the price, 1st/business class want luxury, and sitting elbow to elbow with the riffraff in beat up terminal seating isn't that.
Business/1st class passengers (at least on international flights) almost always have access to lounges, rather than waiting in the gate area. As good as the plane seats are, I'd still rather spend as much lounge time as possible.
I would never consider waiting in a lounge past the start of boarding.
The risk of having to reschedule a $5K flight is to great to justify the reward. If I'm not going to be in the lounge, I'm back to minimizing the time at the gate.
Also, most lounges really aren't very good. AmEx centurion lounge is nice. But like, the United lounge in your average airport is really a few steps down from your typical airport bar.
To be clear, 1st class is about 10x to 35x more expensive than economy for a transatlantic flight (I wouldn't bother with upgrading on any flight I couldn't catch some sleep on).
(BA: ~£200 Economy for LHR-JFK, £2300 to £7300 depending on day 1st Class)
Others have noted other reasons, but I think the biggest reason is that 1st/business class seats are more comfortable than the crappy crowded boarding area. Economy seats, it's the other way around. And as an added bonus, on something like a 787 where the boarding doors are behind business class, boarding first means you're lounging around in the front of the plane being served drinks and addressed by name while behind you the rabble are are being herded like cattle into their tiny seats at the back.
For 3x the price, 1st/business class want luxury, and sitting elbow to elbow with the riffraff in beat up terminal seating isn't that.