I can still hear them chanting "Cambio, cambio, cambio" and it's been a few years. ATMs give hardly anything and charge a large fee per transaction as well, so it's no surprise people go to arbolitos.
You'd also get a better exchange rate from the arbolitos vs the ATMs.
If I remember correctly, it was referred to as the "black rate". If you paid a vendor in USD directly, you got the "blue rate". Both of which were much better rates than the ATM.
