They don't need to. But it's accepted in the community.
Now why they do it.. is very simple. It is very easy to incentivize people to use money on power when you have them see what other people do with money. And with the power being behind loot boxes, you don't know how much money the powerful people have used. So you think small amount of money gets you somewhere. And once you use a bit, you have already justified using money to get power and the next step is to use more money. And once that's justified, what is another $100. Eventually you go out of money or you have the money and end up using thousands. And better yet, people with a lot of money are really competitive and are used to success in life. So getting those people competing against each other is winning the jackpot for the company.
And that is just the basics of it all.