Because it's mostly true.
It's not _always_ true and we shouldn't hold prejudices, but it would be just as wrong (though not as un-PC) to claim that public transit was used equally across income levels, racial/ ethnic checkboxes, etc.
"Americans who are lower-income, black or Hispanic, immigrants or under 50 are especially likely to use public transportation on a regular basis, Pew Research Center data show."
https://www.pewresearch.org/short-reads/2016/04/07/who-relie...
The bigger problem for most folks (not your racist co-worker) is the crime, drug use and general anti-social behavior on public transit. This will vary by city/ transit district, but it's gotten much worse in the last decade (after it had gotten much better for the previous two).