Why do some pros play in 4:3?

Solution 1:

Some people claim it offers a better field of view or gives you an advantage. But mostly it's just down to personal preference. In some cases, players may prefer playing in 4:3 because that's how they played earlier iterations of the Counter-Strike series (such as 1.6).

FOV is not bigger, though. As noted by Decency in their comment, 16:9 offers you a wider FOV, the full 90 degrees, while 4:3 gives a FOV of ~74 degrees.

Solution 2:

I'm not a pro but..

I've played 4:3 because the models are bigger and you're less likely to miss. However, it's a minor difference and as Stark already noticed, it comes down to personal preference.

I've had some games where I didn't see an enemy because of my 4:3 ratio, but my teammates who played CS:GO in 16:9 did.