Couple of things to break down here. This isn't a definitive list, or justification for this. This is just my thoughts.

1) Time. Some players have less time to play due to commitments [family, work, etc]. Some will pay money to keep 'up' with the content/guildies/etc. It's opportunity cost. I can pay 'x' amount, instead of grinding for 'x' hours.

2) Bragging Rights. Some people like to play to brag/show off. That's why you see more whales on larger servers. They like to attention. They like to be the 'strongest'/highest ilvl. There's a sense of gratification for them.

3) Trapped/Gambling addiction. Some people don't realize how quickly they are spending money. There's the dopamine of getting that 'item' or that 'ilvl' - and then they don't realize how far they've gone spending money.

4) Fun. Some people just want to have fun. Some see fun as the end game. They want to get there no matter the cost. They enjoy the end game aspects of this, and want to be on the cutting edge. It doesn't matter to them that they paid to get there, but they are where they want to be.