I think it's because of incentives. For-profit companies are incentivised to maximise profits. There might be resistance against doing so in unethical ways at the start but as the company grows it becomes harder to control until eventually it just does whatever is most profitable, limited only by laws (most of the time). Any CSR or ethical decision becomes a PR stunt to distract from the unethical behaviours.
A lot of not-for-profits also fall into this trap unfortunately as they try to maximise revenue or as they become bloated with administrators.
I think at the founder level - trying to start B-corps, keeping them small-mid and embedding ethics deeply into the culture is the way to go but that is quite different from startup culture.