Mushroom substrate newbie questions?

So in the true spirit of permaculture I’m trying to wrap my head around substrates/grain spawn… for grass loving mushrooms why could we not just chop up a bunch of grass? Instead of using rye or wheat berries? I presume they are less nutrient dense but if you add a bit of composted manure wouldn’t that be the most similar to nature? Or is it just not enough nutrients to get any yield? Too easy to get contaminated? I just wonder if it really is as difficult to grow stuff as they say or is it just about trying to get MAX yield? It just seems like these mushrooms wouldn’t grow in nature if it really was that particular.

I tried growing mushrooms at home using dried banana leaves once, and while it sounded like a good idea, it didn't work as well as I hoped. I also think the case is same with the grass. Grass doesn’t provide the right nutrients for mushrooms to thrive, and it can easily get contaminated with mold or other unwanted fungi. I found that using grains like rye or wheat gives the mushrooms the best chance to grow because they’re more nutrient-rich. Adding compost is a good idea, but it still needs the right balance. Growing mushrooms isn’t super difficult, but getting the best yield does take some care and attention!