Because in order to hear you have to understand
what do you think God meant when he said hearing they may not hear and seeing they might not see
The Greek word akouO has a range of meanings. To hear, as in "I can hear two people talking in the room next door, but I can't make out what they are saying." To listen, as in I am listening, but I don't understand what they are saying. Hearing does not of necessity include understanding.
See is blepO, which also has a range of meanings from merely seeing, as in "I can see a sign up ahead, but I can't make out what it says." and "Now that I am closer, I can see some mandarin characters on that sign ahead, but I can't read Mandarin, so I don't understand what they mean. Seeing does not imply understanding either.
Jesus never cast anyone out who came to him, but he spoke in parables so people would self-select, and those who wanted to understand scripture the way the teachers of the law were teaching it would leave, but those who willing to understand scripture in a new way would stay and listen attentively. Those whose hearts were hardened would hear and see, but would not listen and observe, disinterestedly wandering away and leaving him with the sincere seekers.