Why do people have to pay to get out of jail?

Why do people have to pay to get out of jail?
The right to bail is both afforded and protected by the US Constitution.
If you are eligible for bail, you will be released upon the posting of the proper amount. This is designed to assure your appearance at each and every subsequent appearance in your case.
The court does not keep the bailbond. It is returned to you if you successfully show up at every court appearance to which your are subpoenaed.
Occasionally there are those cases of which part of the sentence is that the perpetrator pays restitution to the plaintiff( (the wronged party). THAT is part of the courts sentence and the court keeps none of the money meant to reimburse the plaintiff.