This is an interesting and deep question! The game theory of life. I don't know the answer or if there is a clear answer, but I imagine there's generally a lot of risk involved in harming others for personal gain, and basic fear of individual or group retaliation (either mob or police) is a significant deterrent.
If you "believe" there's an objective morality, isn't that just another subjective position? You can't justify it, so why don't you abandon the notion and go with your selfish whims?
It's a flawed dichotomy. Morality is in your perception like how you know to respect other people's personal spheres. It's not based on rule based rationality or some axiomatic selfishness.
If you want to sleep well at night, you refrain from screwing people over in terrible ways. Probably you screw people over to the extent that you can get away with it in a moral-cultural sense, unless you're particularly saintly.
Genuine question: What do people who believe this use as their reason for refraining from harming others for personal gain? I understand that simple intuitive preferences against seeing others suffer will often work, but what do you do about instances where either the rewards of screwing someone over are very great or where your intuition tells you that you would get a lot of pleasure from seeing someone in pain?
I know that I don't really have anything to support my belief that there exists an objective morality which is hard-to-determine.