It's because people often use logic like that as justification for spending, which doesn't make sense

It makes perfect sense from a long-term perspective. Obviously you buy because you want what you get from it, but Genshin is topping the charts and that means 2 things: It's not going anywhere anytime soon, and more games like Genshin will be made. Genshin itself is a perfect example of this, people bought BOTW so Genshin took heavy inspiration from BOTW because it wanted success too. 4 years from now we'll absolutely see more games like Genshin that want to make as much money as Genshin does.

Games that don't make money don't get much free DLC, and they certainly don't get high quality imitators.