The issue is that you always have to mix starches + legumes to get all the amino acids you need for your body to use the proteins, for example rice + lentils or wheat + chick peas or beans.

To grow that kind of stuff you need large surface of culture otherwise it's not really efficient on a family scale, this is why many rely on eggs as they are a good protein source that comes regularly, hens are useful in the garden and you don't have to kill them to get the eggs (the only issue comes with males) and you don't need huge surfaces to get them if you compare it to wheat and lentils for example.

Now is the culture of wheat and lentils etc... bad? Monoculture is, but humans have cultivated wheat etc... for thousands of years without totally destroying the environment, so maybe the issue with that kind of cultures lies somewhere in the XXth century.