Roth 401(k)s (and traditional IRAs) do exist, you know...
For taxation purposes, traditional tax-exempt IRAs are equivalent to 401(k)s, and Roth 401(k)s are equivalent to Roth IRAs. Whenever someone refers to contributing to one type of account or the other over another type of account, the ones that are equivalent are... equivalent(for taxation purposes).
Because the entire thesis of this thread is that it is better to contribute to a 401k and other pretax accounts before contributing to a Roth under nearly all circumstances(other than at the beginning of your career) that don't involve massive tax raises in the future.