Once a will is signed, it is put in a drawer and sits there for hopefully a very long time. Once you die, the family goes through your important papers and finds it.
Unfortunately, no one really knows that you wrote a will except for a select few. No one knows if that is truly your will and no one knows if you subsequently wrote a different one. You are now asking the banks, investment firms, and title companies to rely upon some documents you simply found in a drawer. That is a lot to ask so they will tell you to take that will to a probate court to make sure it is valid.
A will by its very nature needs to be probated through the estate administration process because the one person who could tell us if it is valid or not is deceased. We now need a court to tell the financial world that the will is valid so that they can rely upon it. Otherwise, we have no idea if it was forged, signed under duress or a later version is available.
Probate sorts all of these issues out which is why it is a very public affair. Otherwise, we would not know if someone is sitting on a later date will of the decedent.