The thing with any material-based economy is that there is a limited amount of resources. If there are too many people, the share is too small. Acting selfishly is a way to increase your cut by stealing the cut from others.

Cryptos are not based on gold or oil. They come into existense by providing services and aid to others. Yes, they are abused through whaling and vote manipulation, but just like everything we do, there needs to be a sense of reward. We are motivated by knowing there is something good at the end. Altruism works in the same way.

Since steemit rewards you with seemingly infinite tokens, helping others is easier than trying to steal from others and altruism has a higher chance of happening.