Because cash, for better or worse, is how our society determines "need". It's by no means perfect since the wealthy have an advantage but what's the other option? A panel analysts to review who really "needs" that seat?
Demand pricing (plus full fare flexible tickets and first/business class) is what helps less affluent people being able to afford a ticket at all. So instead of everyone having to pay a flat $600 for a seat, people with flexible travel dates can buy early and pay $300. They'll lose some amenities (like legroom, food, "free" baggage, etc), but for some people, that can make the difference between being able to afford a ticket at all or not.
If the plane is so full that the airline can triple the price over what they cost 2 weeks ago, it's likely that there would have been no seats at any price when I wanted one if they kept prices steady at $400.
I'm by no means wealthy and $1300 was painful, yet still I was thankful that I could get a seat at all for a very time sensitive trip.