9 out of 10 startups fail? Where is this statistic from? Every single I see this troupe repeated I wonder why people don’t question it.

These are, as far as I know, well tracked datasets. For the US the Bureau of Labor statistics tracks each annual cohort of new companies and the attrition over time: https://www.bls.gov/bdm/us_age_naics_00_table7.txt

So for example the first chunk there, cohort of companies that started March 1994, there were 13% still operating 10 years later in 2024.