I agree with you, and what you are proposing is closer to the Canadian model: as long as you have earned income, you can open a tax advantaged account (RRSP) with any provider. You contribute up to your maximum, and get a tax refund to even things out.
You don't have all the horrible plans and hideous fees in Canada (the mutual funds themselves are not great, but not as bad as some American 401ks I've seen)
My understanding is that it is VERY profitable to manage employer plans and tack on fees, so I imagine there would be a lot of political push-back from well connected industries if anything tried to change in the US.