I think the perception of risk is what is off more than anything. LA metro for example saw about 180 violent crimes on it last year. For a system that sees almost a third of a billion boarding a year, the risk of violent crime happening is so low it doesn't make sense to consider. I expect the crime rate is similarly low on other transit systems.
That said the risk of crazy people ruining your day for no reason is not low. Ultimately the problem isn’t transit (rather it’s part of the solution), the problem is that the US social fabric is messed up.
https://libraryarchives.metro.net/DPGTL/CrimeStatReports/202...