Why are most people in Fortnite only 3 options for full screen resolutions and for me 5 means beyond 4k. Most of the players take the highest resolution for them, corresponding to 1920x1080, but it looks exactly worse for me because it has to do with the fact that I have more options. Why is that.
Which "per player" do you mean?
Yes, because the game looks at how strong your hardware is and adjusts the possibilities
Your monitor has a certain resolution. Fortnite actually chooses the highest available. With you it would be 4k or UHD now have no plan which of the two.
Because you have a 4K monitor and most pro gamers have an FHD 1920x1080p monitor with 240Hz
Games show the possibilities that your monitor can show.
Most people have a Full HD monitor, which is why it is 1920x1080. You seem to have a 4K monitor. For this reason, you can select resolutions up to 3840x2160.