Most consider flying first class a luxury reserved for the wealthy or those who don't fly very often. The majority of flyers could never consider a first class ticket a responsible purchase.
Usually because it's ridiculously expensive. Would I pay 20-50% more for a first class seat? Probably. Would I spend 2-3x more? No. I'm still going to be miserable, the flight is still going to take just as long, and once it's over I'll be out all that extra money.
Those who fly a lot may get frequent upgrades through an airline's loyalty program. Some programs will upgrade you automatically if there are any seats available.