Because players are not the smartest people, neither particularly good at negotiating. So they hire people to do the negotiations for them. In almost every case, that brings them more money than if they did the negotiation all by themselves.
Which in turn means that the agents who are good at bringing big money to their clients, get their share of pie too. With the transfers and salaries exploding, it is normal that the agent fees have exploded too. And if the players can bear the risk of running down their contract (which might be very bad if they get injured in the last year), both players’ salaries and agent fees are gonna increase.
From my point of view, there is absolutely nothing wrong with this.