The top 5% household income in the US starts at ~$270k, which leaves at least $100k for investing after taxes and ordinary expenses. Combine this with US investment market performance e.g. the S&P500 over the last decade, and it is straightforward how someone with a good income might arrive at $2.5M in a relatively small number of years starting from zero.

Americans have notoriously poor investment/saving habits, which obscures the fact that it is remarkably easy to accumulate liquid wealth in the US if you are a diligent saver. The median US household has enough investable income, after all ordinary life expenses, to accumulate a couple million dollars over a typical career. High earners can easily save 10x that amount.