Private practices of a solo or small group of physicians aren't the same thing as large corporate ER's/Hospitals. Speaking as a physician, if you screw your patients with large surprise bills, they're going to quickly lambast you online and switch to another physician. Not exactly the best way to build a successful business. Private practices generally are for specialties that typically deal with elective complaints, where the problems aren't urgent and you have a choice of who to see. Whereas if you're a large ER or hospital, most if not all of your patient's are in some sort of emergency or urgent condition and don't have any choice but to be in your hospital, without much say in the matter.

We get little to no training on how to bill for our services in medical school or residency. Most of the time we have to get lectures on how not to underbill for what we're doing.

The other point to consider is that fraudulent billing is serous. The fines are repercussions are massive, including being cut off and banned from those insurers or Medicare/Medicaid entirely.