Evidently, some folks have (or had) employers who offered generous matching, and the "mega" option. Also we have the ubiquitous advantage of married couples vs. singles. That explains the large percentage in tax-deferred vs. straight-up taxable.
We also note that tax-deferred vehicles really burgeoned over the past 20 years or so. Older folks might remember the Tax Reform Act of 1986, which really limited IRA contributions for a broad class of employees. Roths didn't start until 1998. So, for the latter half of the 1980s, and most of the 1990s, a whole generation of folks "grew up" either with skepticism towards tax-deferred options, or didn't have them. That was also the overlap period, when defined-benefit pensions started going way... but that's a topic for another thread.
To the OP's point, I'd maximize the backdoor Roth to the extent possible, ignore traditional IRAs (other than holding tank for the annual Roth back-door) and only contribute to the 401K to garner all possible matching. The annual remainder goes into taxable.