I think a large factor is because so many people use it. A lot of people come to self hosting without much knowledge and just copy configs etc. from a Tutorial. Those tutorials will 90% of the time use Apache or nginx. I remember back when I set up my first servers I mostly followed instructions and copied configs. Years later I understood I had set up Apache with virtual hosts and what that means/how it works but it might as well just have been nginx.

As for why so many people use these two I think it also has to do with "adoption" in another way. Back before nginx Apache was the standard everything else was "different". Then nginx appeared to solve the Problems of Apache and then there were 2 ... These days you can probably do anything you want/need with the 2 servers so no reason to use anything else.

Professionaly I usually use either HAProxy and Apache or Nginx (or sometimes HAProxy and Nginx) but if there are special requirements that might change.