Because no one has yet had a need for them and created the footprint. A quick search doesn’t show anyone with a qfn52 Fritzing part so you (or someone) would need to make one. A quick look in the adafruit fritzing library doesn’t show the bare chip (they have a breakout board I think).
Edit: are you sure you want a 52 pin qfn the Holtek data sheet shows a lqfp52 which seems to be a different layout than the qfn 52 (~7mm by 7mm for qfn vs
~14mm by 14mm for lqfp52 in the Holtek data sheet).