Of all your matches, those with lower than average attractiveness are more likely to message you first (on apps where women aren't always required to message first).

This leads to a negative correlation between perceived match value and them initiating the convo.

Some (most?) people would go further and reverse the causation, assigning lower values to matches that message first.

So, it's an obvious move to only message the bare minimum 'hi' because any further investment would be a signal that you perceive the match to be unbalanced with your own value being lower, and are trying to compensate for that..