How is it that simply having a large amount of cash on you makes you criminally liable?

My question comes from the fact that the reason some criminals need to commit money laundering is cuz having a very large amount of cash on you can make people suspicious of you and can also make law enforcement think that you obtained the money illegally. Well, can someone please explain to me why simply having a lot of cash on you gives people the right to be suspicious of you? What gives law enforcement the right to harass you simply cuz you have a lot of cash in your piggy bank?