For the same reason there is usually a 7:1 Palestinian versus Israeli ratio of deaths during the fighting.
Palestinians are far more willing to suffer and die and send others out to suffer and die for their cause than Israelis are. Hundreds of Palestinians languishing in Israeli jails is only a problem for them in the context of less fighters. Hamas is never under any pressure to get their prisoners freed from the prisoners families, whereas the Israeli leaders are always crazy intense pressure to get the hostages freed. Therefore Hamas has the upper hand in the negotiations which it exploits to the fullest.
I’m not saying the exchanges are the right thing to do. I’m just saying why they happen