Why do some pros play in 4:3?

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.