Serious question, why do people feel motivated to grind gold in video games, but not money in real life?

Instant gratification on rails, essentially.

On one hand you have slots/roulette/craps gambling where payout or loss is immediate. Next come candy crush style games where time invested per session is minimal but extant, but reward continues to stream with flashy graphics for accomplishing subgoals. Next come longer-term games like 4x or tower defence where there's progression or some sort (often available to skill or how much time you have to spend) where payoff is delayed to the end but there are still progress rewards like new technology or beating a wave of enemies. Finally RPGs and MMOs where there is (often) no final payout but leveling up, completing quests, acquiring special items, etc. happens at a pace that rewards time spent. At no time is the next step mysterious; there's a journal with next steps which will each provide their own rewards.

Contrast that with real life where reward is on a fixed schedule (N paychecks a month) with no short-term feedback on performance. At best a short "good work" reward fairly rarely from management. There are subtler rewards people get like socialization or status, but they aren't on a quest journal and they arrive less frequently for most people.

I think for extroverts in high social contact jobs it may actually be as rewarding as gaming, e.g. direct sales.

Contractors and spot workers (day laborers, etc.) also get a bit more frequent reward feedback, but far fewer rails unless it's a very canned pipeline of clients.

Contrast with the ancestral environment where hunting or gathering had minutes or hours between reward feedback and our natural senses and instincts provided a lot of the rails (hunger, visual attention to animals and edible plants, potential mates).

Games are designed to incentivize you to stay in them. Real life has you as a captive audience, and to the extent that it's "designed", it is designed by people also playing the game, against you who have no incentive to encourage you to try harder to beat them.

I like that I can hop on at any time of day, make some progress, hop off any time. I don't have to wake with an alarm and stay out gold farming for 8 hours with 2 or 3 short breaks. I don't have to leave the house. I don't have to talk to anyone. I grew up with MMOs like Graal Online, RuneScape, MapleStory, etc. I have been extremely disappointed by the real world's version of jobs and money. Gold in a game often feels more useful as well. I have actual needs for it. Real life by contrast is vaguely like "work forever and keep making money because that's just what you're supposed to do".

in the game, the success is guaranteed. Time in = gold out.

IRL, success is NOT guaranteed. time in does not necessarily yield additional income.

in the former case, the reward pathway in the brain is satisfied, so why risk it?

These are the same thing often times. Venezuelans made more money bottling GP in RuneScape than they did in their home country working real jobs.