Why does my chopped beef come out too tough?
As Raditz_35 mentioned in his comment you prepare the meat in different way than you want to. Yes, in curry and stew/gulash you use low quality (it's not actually low quality per se, it's just more dense and more chewy onto itself) meat. And then you STEW the meat for few hours.

You try to fry it for few minutes. 

If you want to have a stir-fry you need to cut the meat even thinner. Think paper thin. Then you fry on very high heat with high smoking point fat.
If you want to make a stew/curry you need to follow the recipe. Almost all recipes I've encountered counted the time of in hours or stated "simmer until beef is tender". 

Browning the meat on pan before stewing is important part as it keep all moisture inside the meat so it' not come out dry.