The challenge is three-fold.

First, existing dams tend to have low head heights, an average of 70 meters. As such, the value proposition is lower.

Second, there is frequently no good location below the dam for a reservoir of significant size. They tend to be sited on rivers, not lakes.

Third, electricity has to flow to the dam from wind and solar sites, and typically that transmission doesn't exist.

I'm more of a fan of pivoting existing dams to be less baseload and more peaker and back up energy supplies. Let nature fill them, and manage them for balancing, not baseload.