Except that the point of overbooking is to allow UA to sell expensive fares at the cost of potentially giving a voucher to someone who has purchased cheap fares. There's little incentive to offer a discount on a flight that's already full in order to create a VDB situation. You may as well just stop selling tickets. :-)

But still, suppose UA can tell months in advance which flights are likely to be oversold (they frequently can). You still run into the perception problem, as customers simply don't read the fine print. (They don't even tend to read the large, bold print that requires an explicit acknowledgment ). People are not going to remember that UA offered them a discount to be VDB'ed in advance. They're just going to remember that UA was the cheapest, but then they missed their sister's wedding / spring break trip / cruise / etc. because they were denied boarding.

Honestly, the current system is better for everyone (well, it'd be better still if they went back to the previous system where VDBs were common). I volunteer whenever I have the opportunity and my plans are flexible, but I don't always know how flexible my plans will be on the day of travel when I buy the ticket. And I would never just put myself at the whims of a UA auto-rebook. Whenever I attempt to get a VDB, I always know what my alternative options are (UA and non-UA) so that I can find something that works for me. If I'd already agreed to be bumped from the flight, I have no leverage and simply have to hope that the gate agent is in a good mood.