Because these criminals are using VOIP to spoof numbers or prepaid cell phones which can be tossed, making them next to impossible to trace. The fact they are also operating out of corrupt and apathetic countries and out of the reach of US jurisdiction does not help, either.

I've received two different voicemails with this scam. Today I believe I got that same name, "officer Melvin," as mentioned in a few of these posts (I thought it sounded like officer Melnick). Anyway, the same script, same accent, same scam. I called and left a message at Treasury Inspector General for Tax Administration, www.tigta.gov and also filled out the tigta.gov online complaint form. After that I Googled and found this site. What a relief.

Caller ID showed the same number the scammers wanted me to call: 571-210-5551.

They called me at 5:30 AM at home, with the same message!

I received two phone messages from a Officer Melvin with a similar spiel that they were about to issue a warrant for my arrest based on tax fraud, but if I called them back I could give them relevant information to clear the matter. Otherwise, they were going down to the federal courthouse to get a warrant for my arrest.  I have not bothered to call them, but I might do it to see their reaction.  

hope their sweaty shop catches fire