No this is actually true. Most stocks do go down (or at least only match much less risky investments like short-term bonds), but the stock market goes up because some stocks go up a lot. This is why all investors diversify. For example, the Russel 3000 Index goes up ~10% a year because they remove bad stocks and replace them with ones that seem better. But, employment at a single company with stock-based compensation is a concentrated bet on that single stock during the employment timeframe.

Recently learned this from the excellent and very pithy Michael Batnick blog https://theirrelevantinvestor.com/2020/09/10/most-stocks-suc... who was citing https://wpcarey.asu.edu/department-finance/faculty-research/...