Why do governments have to simultaneously be in the business of (1) securing people’s property and (2) creating money and regulating the monetary supply?
Because regulating it is an important part of securing it. They're not separate things.
If I am a victim of fraud and lose my money, I have the concept of the rule of law to help me get my money back. In a decentralized world there is no way to get my money back.
Besides, it is the government's job to help society function well, in part by making the economy successful. Money is just a tool to do that.
They have to supply the money so that they can have something which can be collected with reliable value for taxes. If your government allows you to pay taxes with monopoly money, then Hasbro becomes King. If the government supplies the money, there is no external organization which can inflate the supply to gain control.
