Vast majority of the population wants to travel more but, of all things, the cost of a plane ticket is holding them back? No, this is not true.
I travel for pleasure about once a year. I can travel more if I wanted to, finances don't hold me back, but once a year is enough for me. Traveling for vacation is usually pretty fun but at the same time just stressful enough that I wouldn't want to do it more than once a year. Airplane travel is really inconvenient.
Plane tickets are just a small part of the cost of a vacation so a free ticket isn't magically going to make them afford extra travel. United isn't paying them for their hotel, rental car, and activities they are going to do when they go on this extra vacation.
Even if you do make use of the vouchers, you better hope that United flies to that place you want to go and you have enough time off saved up at work and your schedule works out to use that voucher before it expires. I know you get a year but people have schedules and usually only have windows where it works out to take vacation.
If you are a business traveler you better hope you actually like traveling for pleasure.