Why cant you susbtitute oat flour for wheat flour 1:1?

I don't know what you mean by avenin being "similar", but it doesn't behave like gluten at all. Oat flour behaves like any other gluten-free flour and is a poor substitute for wheat flour. You can only use it in recipes which are specifically engineered for gluten-free flours. If you try using it in recipes which rely on gluten, you will fail for certain.

It is somewhat interchangeable with some other gluten-free flours like chestnut or sorghum flour (also buckwheat if you don't need the flavor profile), but gluten-free recipes being fickle, you always run a risk of failure, unless you have a pretty forgiving recipe like crepes. It is not a good substitute for nut-based flours, and is a mealy, not waxy starch.