These are girls who are attracted to guys who are really tall.
That is what their preference is, typically across the board.
There is a good chance that these girls who want guys who are 6 ft tall or more on dating sites tend to date guys who are that tall in their real lives.
They don’t believe that those guys are a good match for them.
Though dating sites may match you with girls, that doesn’t mean that the girls will feel that you are a true match for them.
Indeed, the matchmaking algorithm may have matched you with girls that were seemingly compatible to you, based on the information that the matchmaking algorithm gathered from your dating profile, but the girls may not agree with that assessment.