I think Errant Signal has a great video [1] dissecting the appeal and breadth of content in Fortnite. It's not my cup of tea, but I certainly don't think you can call it "shallow". On the other side, Folding Ideas has a great video [2] (that the Errant Signal video references) dissecting the monetization systems in Fortnite and their negative aspects.
Can you expand on what about it is so interesting to you? My only experience with Fortnite is watching my 13 year old brother practice crazy fast building, and every time I can't help but wonder why they'd rather play that than Apex Legends. I sincerely don't understand.
Grinding fast builds helps you when you're fighting. :) Boxing someone and then editing a hole to shoot them is sort of "top tier" gameplay.
In general, not only do you have to shoot people as in a typical FPS, but you have additional defenses you can put up (builds), you can use those builds for mobility and gaining height quickly, Epic constantly rotates the weapon set, they add/remove mobility items all the time which forces you to change how you move across the map, and the map changes.
And then when I just want to chill there are all sorts of non-fight challenges and (relatively shallow) lore to chase down.
It's never not interesting, and if you like grinding and seeing your own skills and reaction times improve, it's all there
edit: I should link some videos.
1. One of the best players in the game right now getting 25+ kills in ranked play (ignore the rich kid chatter, he's 16 and already made nearly a million in comps which makes him insufferable sometimes): https://www.youtube.com/watch?v=FhTCEEeLvdk
Hate to take offense to this, but as someone who plays Fortnite competitively, it is absolutely not shallow. It's so much more than just another FPS.