VoIP (Voice over IP) has made it easy for a single computer to place tons of calls simultaneously. Lack of regulation by the FCC (Federal Communications Commission), and the phone companies, has made it easy for people to get away with doing this. Real companies like Wells Fargo will use robocallers to harass customers with errant accounts, and scammers use it to call random people.
Caller ID spoofing has also become a huge issue since it’s really easy with VoIP. The Robocaller simply chooses a random phone number that uses the same area code as your number. So if your number is 626-555-5555, it will call you using a number like 626-555-1234, making it seem like a local company or person is calling you. All it has to do is tell the phone company this is their number, no sort of authentication goes on.