They made two, actually, the Gew 41 and the Gew 43.
The 41 wasn't particularly successful, but the G43 was a very good design.
As for why the Germans didn't develop a rifle before the war for mass issue...
1. They were rebuilding an army largely from scratch, and the K98k was available, and proven. Semi-autos weren't really available, and certainly weren't proven at that time.
2. Given that they were trying to get their military back online, there wasn't a lot of manufacturing bandwidth left over to start full-scale production of an entirely new rifle.
3. It's very likely that a semi-auto rifle would have been fielded and in wide-scale circulation had Hitler kept to his original war plans, the ones that he kept telling his military of no war until mid to late 1940s.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.