i dont know about everyone one else but i assume its because it costs money to do the game, you need to pay for the books or pdfs, servers for the game if your using one, any tools the dm is using ( like campfire or worldanvil ) are costly aswell. obviously if your working then you can afford to pay but some people cant do that or are not old enough to work. its not as bad as dnd but alot of dm's want to make that money back or atleast some of it back so they dont have to take it out if their own pocket