The fact is that retirement planning takes a combination of discipline, knowledge, and available cash that very few have. The 401k provides nice fees to financial companies, and is good if you want portability, but it is poorly suited to the vast majority of people who lack the discretionary cash and knowledge to make good use of it. Combine this with the reduction in overall employee compensation when defined benefit plans were replaced by DC plans with an employer contribution that was worth far less to the employee than the old DB plans and it's small wonder that US retirement accounts are so small.
The UK faces a similar issue of low retirement savings rates and to address this they have done two things.
They have reformed their equivalent of SS by removing any connection to earnings. High earners used to get a bigger monthly check than lower income people, but now the retirement benefit is calculated only on number of years you've worked. If you reach 30 years of contributions you get maximum benefit and everyone gets exactly the same. The contributions tax remains a percentage of earnings paid by both the employee and employer so you pay more the more you earn, similar to FICA, but you don't get a bigger benefit. The result is that low income people have seen a big increase in their benefit and high earners have seen it reduced. The reasoning is that low income people rely more on the state benefit than high earners so a higher state pension to them will reduce poverty and the high earners won't miss the lower state pension because they have additional sources of retirement income.
Secondly the UK has introduced a mandatory DC pension plan that every employer must offer unless they have an equivalent or better company plan. The company must pay into this, but the employee has the option to opt out. It's called the National Employment Savings Trust (NEST)
NEST home | UK employer pension scheme | NEST pensions
However, I don't see these approaches being implemented in the US because of the egalitarian aspects of the first and the mandate of the second.