The classical argument is that if God exists, then objective moral values exist, as they arise from God. But, without God, there is no authority from which objective moral value can arise.

More modern takes might consider if thermodynamics or genetics/memetics might give rise to values that out compete others and thereby are objectively better than outcompeted values.

Even if you land with subjective values, and freedom to associate with a tribe that shares these same values, you do still have to worry about other tribes being more successful than yours, pushing your tribe and its values into irrelevance and extinction over time.

I guess I have just convinced myself that objective moral values do exist, but only as a result of "survival of the fittest"