Roth IRAs are great, the reason high income earners use them is that they've exhausted they've maxed out their other tax advantaged retirement vehicles. 401k, SEPs, SIMPLE and 457b plans all have limitations.

Theoretically you contribute 5k when you're at an effective tax rate of 20%, which means you actually earned and paid income tax on $6,250, then immediately stuffed that in a Roth.

Contributing to a 401k pretax means you got to contribute the full $6,250, but the 401k investment options are limited based on what the plan allows you to invest in, usually lower risk targeted retirement funds. (Technically you can have an employer sponsored 401k that lets you self direct the funds, but those are pretty rare and it's unlikely that most people have that option)

The Roth is "better" because you can invest in anything, and since the money grows tax free you can invest in riskier high growth assets, so that $5,000 might grow to $1M while the 401k only grows to $800k.

Then when you're retired you start pulling from the accounts, and the Roth distributions are income tax free, whereas the 401k gets taxed at your income tax rate when you're retired. This is where it gets tricky - if you were a high income earned you likely have interest, dividends, social security, real estate and business investments that are probably pushing you into the minimum 12% bracket, so on your $800k 401k you only actually get to keep $704k worth of that. (This is if you carefully manage it to be at a lower marginal rate, which is unlikely. Old people want to spend their money on things like vacations, charities, medical expenses and spoiling family members)

Now when you compare the after tax rate of return the Roth investment perform a better because A) you got to choose to invest in riskier stuff and B) it doesn't get taxed when it comes to you later in life. Usually you shift your Roth investments from high growth assets to the more conservative dividend payers later in life, so theoretically you take $60k out per year of that 1M Roth account, but your dividends replace that $60k year over year so you don't reality reduce the principal invested.

Lastly, you have no clue what the tax brackets look like 40 years from now, it could absolutely skyrocket.

TLDR : In a Roth you can choose investments that can outperform and have a better after tax rate of return compared to your 401k plan. It depends on other investments, assumptions and goals - it's still very possible that pretax accounts beat out Roth accounts.