How many 20-year-olds could afford to invest $300 a month even if they’re being paid a middle class wage?
If you're making median income outside places like LA/NYC/SF and don't have children, it should be pretty affordable.
The place I work at doesn't offer a 401k either (technically I'm self-employed/contract worker, so I do offer myself a 401k), but even if it doesn't and you're not self-employed, IRAs have a $6500/year limit and that's something you just open yourself. And you can just open a brokerage account if you want more. Unfortunately, most people don't know these kinds of things, especially not at 20yo. There's definitely a education gap, which is a serious problem. And few people in their early 20s make the median wage and even if they are making that much, many are still buying lots of basic durable goods like furniture and kitchenware. So using 20 as a starting point is probably a little too optimistic...