I have flown all over the world in first class: Lufthansa, ANA, Asiana, United, Singapore, TWA, Cathay, and JAL. All on miles, never paid for it. For 16 years, until the company I worked for went out of business, I flew well over 100,000 miles a year, all domestic. The company only paid for coach, but as a super-elite you get upgraded a lot. You also get a 100% bonus on your miles so that's a minimum of 200,000 earned miles a year, so, when you add promos and bonuses, you can see how one could earn enough to fly first class for free.
IME, non-US carriers are very stingy with their F award availability. In order to get 2 F award tix to Asia on Cathay/JAL for travel next month, I had to book 330 days in advance. Flying from MUC-EWR last year in Lufthansa F, we were the only two pax, so that means they sold zero tickets. On a Singapore flight LAX-NRT, there were about 4 people in F, (which really frustrated me since I was unable to get my partner an award ticket, and he had to fly the next day). On US carriers, ITOH, international first and biz are always full, I would guess with free and paid upgrades. Of course, their product can't compare.
Just for fun, I put together an Expedia trip JFK-HKG-NRT-JFK on Cathay and JAL just to see how much our award tix were worth. $24,000....EACH. I would guess almost nobody pays that. I would speculate CEOs who fly F have a discount with the airline, due to large volume, and celebrities have a contract that stipulates they get to fly First.
Lastly, when people ask why I burn my miles for First and not Business, I always say that I can afford Coach, hopefully, someday I will be able to afford Business, but I will never, ever be able to pay cash for a First Class ticket. But it sure is an amazing way to travel.
BTW: The job I have now has zero travel so this is my last hurrah. I now have about 2000 United miles left and 38 (not 38000, just 38) AA miles left. Next trip, I'll be in Coach.