>>58280209FIRE specifically is about early retirement, 250k are not gonna last you in the US if you intend to retire at an age of 30-50. Fat FIRE is a version of FIRE where you aim to maintain a costly lifestyle in retirement (like the one most Hololive girls are used to), for which 2.5m+ isn't unreasonable at all - that's 125k/year in capital gains (before tax) assuming a modest 5% interest rate. Obviously you can start cannibalizing your initial investment once you grow old to increase your disposable income, but that's not an option before you're like 70 (and even that assumes that you're fine leaving less/nothing to your children).
Of course, if you're willing to move to SEA or Eastern Europe or somewhere else with low cost of living, you can get by with significantly less money.