Why is my cpu too weak?

Why is my ryzen 5 2400g too weak to have continuously ü140fps (in Fortnite)? With an rx 5700xt (wanted to get me a new cpu)

The CPU is basically an R5 1400 without solder, with a graphics unit and a slightly better RAM controller. It's just no longer the newest and only gives your graphics card 8 PCIe 3.0 lanes, although it should run with 16 PCIe 4.0 lanes if possible. So the graphics card only has a quarter of the actually possible bandwidth to the CPU

Would that look better with a ryzen 7 3700x

Clear.

Yes, best in combination with a B550 board if you can afford it