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.
Having to send rejection messages to every guy that messages them on online dating sites may be practically impossible.
Many women would literally have to spend hours just sending rejection messages and while they are sending those rejection messages, they are receiving even more messages from men.