https://ucr.fbi.gov/crime-in-the-u.s/2017/crime-in-the-u.s.-2017/topic-pages/clearances

Okay, this is the exact same data by the FBI. Still 40% chance to get away with murder and still not accounting the victims that were never discovered.

Don't kid yourself, few persons would turn themselves in immediately. Most would try to find excuses or a cover up first. If they get into a situation where escaping any sentence is unavoidable, they might turn themselves in.

Now add to this that Polo was a heavily spoiled child. He might had a very bad childhood and has psychological problems (anxiety, panic attacks), but he still nearly always got what he wanted. If there is a group of persons that could murder someone and get away with it it's the 1%. The mum that was always so pendantic would've probably even helped him with that. So it's even more unlikely, that someone like Polo gets caught.