United and Delta tried during the pandemic to improve social distancing [1] but switched back once travel came back, and I'm guessing this kind of boarding ended up being slower. It turns out the Southwest way (random boarding) was still the fastest.

>Loading back to front just moves the line inside the plane, but is not significantly faster than loading from the front to the back.[2]

[1]: https://thepointsguy.com/news/back-to-front-boarding-coronav...

[2]: https://thepointsguy.com/2012/10/travel-science-improving-ai...