Something else to consider is that VX's F product is both more limited and far more improved than a "normal" F product (while VX's Main Cabin Select is on par with most airlines' F product in terms of seat pitch).

In a sense, VX offers a "true" F product and (with a few exceptions) other airlines do not. For a handy example, on the A320, Virgin's seats are as follows (pitch/width/number):
First: 55"/21"/8 seats
MCS: 38"/17.7"/12 seats
MC: 32"/17.7"/129 seats
Total: 149 seats

For Delta, same plane:
First: 36"/21"/12 seats
"Comfort": 34"/17.2"/18 seats
Economy: 31-32"/17.2"/120 seats
Total: 150 seats

United, same plane:
First: 39"/20.5"/12 seats
"Economy Plus": 35"/17.7"/42 seats
Economy: 30"/17.7"/96 seats
Total: 150 seats

Jetblue, same plane:
Even More Space: 38-39"/17.8"/42 seats
Economy: 34"/17.8"/108 seats
(I think this is slowly being reduced in line with other airlines)
Note: On Jetblue's Transcon services, they have a First/Suites option.

American does not fly the A320. They do fly the A319, however:
First: 38"/21"/8 seats
Main Cabin Extra: 34"/17.7"/18 seats
Economy: 30"/17.7"/102 seats

Southwest has one class of service and so I'm not including them in this.

TL;DR: Virgin's Main Cabin Select (which auto-upgrades do trigger) is equivalent to regular "First" on any of the main US carriers' normal services. Virgin's First is not quite on par with the Transcon First/Business services on their premium products, but Virgin offers their First on every route and I don't think the other airlines are particularly keen on handing out free upgrades to the "better" first.

To put this in more blunt terms, the reason most airlines will comp folks into First is that First isn't really First. Conversely, Virgin's First is a "proper" first.