Mods are rarely as advanced as each season of Fortnite. And even then, the developers typically ask for donations for that, as seen with the multiplayer mod for Skyrim. Most would not have enough spare time on that unless they're still in college. Most games don't seem to get any mods, so your premise is true only for some lucky games like The Witcher 3 and the GTA series. And in every one of those games, you can only hope someone likes what you like enough to work to create that, and then others still don't see that interest, as explained in the other comment.
Your interest has been exploited for profit. It's leverage, to trick you into spending more than you would, if they sold games honestly as a product or a subscription service.
I haven't spent a penny on Fortnite but I have a huge number of items from every battle pass and more. As I was saying, it respects people's time spent playing and rewards dedicated players like me.
And to your other point, games like GTA V wouldn't be supported to this day if not for people's interest and spending.