Yeah. I think there are so many hypotheses we could test if professional players were willing to do these experiments.

For example, it used to be popular among competitive CS players to use 4:3 resolutions on 16:9 monitors. Since the target’s vertical position is much more predictable than its horizontal position, it’s supposedly easier to aim if the image is stretched wide.

But these games only presented 4:3 options at low resolutions. This might have introduced the notion that low resolutions provide an advantage in general.

It's become less common to play in 4:3 over the years. The additional peripheral vision has become too obvious a benefit to not make use of.

It's probably mostly a habit thing. Most of the pros started to play these games back in the 90s, when 4:3 was the standard. Add in that playing all the low resolution options are 4:3 (and the 16:9 equivalents will add resolutions rather than take it away; the 1080p is usually 16:9, so 1920x1080, but 4:3 1080p does exist, and it's 1440x1080, which is a lower resolution), it's no wonder 4:3 stuck around as long as it did.