Taxes are the least soul-sucking aspect of this. I've spent an unreasonable amount of my life playing MMOs, especially the Korean ones, where most of the time the goal is to make (in game) money to get strong enough to take on the next piece of content. This is fun because it works like a version of capitalism without, well, everything else. Seeing numbers go up is fun, be it my character's Critical Damage or the funny number at the bottom of my inventory, and having a safe environment where that can be done at my leisure is fun.

But here's the catch: it doesn't stop there. Any of these games where progression is only gated by in-game money and player trading is enabled, will eventually grow an underground market of RMT (real-money trading). The second you're aware of this, it becomes impossible to think of the in-game money in a vacuum. Suddenly you go from playing for 8 hours to make 10 Million Mesos / Zenny / Alz / Whatever to working for 8 hours to make less than half you would working at McDonalds and trading that money for in-game money. This absolutely destroys any enjoyment you could possibly get out of the game, as it is no longer a game.

This isn't even to mention the people who literally automate the game to play itself to make passive income through hacks / bots, though admittedly that can be fun if you're the type of person who's drawn to breaking computers.

Tangent: my latest discovery solves this by just completely axing trading between players. You can collaborate in bosses for loot that gets split between everyone, but you can't just give anyone money directly. It's great!