Because you can? It looks like a great project for getting started with nontrivial FPGA design.

Programming-wise I'd say full FPGA is less useful than QMK. Doing a direct 1:1 mapping from key inputs to USB HID report isn't too bad to do in an FPGA, but dynamic behavior like macros, layers, leader keys, mod tap, auto shift, and so on are significantly easier to implement in a regular programming language. If you want flexibility you're basically forced to have your FPGA run a soft core, so at that point why not go for a regular MCU?

In theory you could make an argument for lower latency, but that doesn't really apply when you're limited by USB 2's 1000Hz polling rate while some off-the-shelf MCU-based keyboards use USB 3 for 8000Hz polling.