By law, any public company in the US (and most other countries) are required to have a financial statement audit. I am sure there are instances where it is not required, but substantially all private companies that have debt are required per their credit agreement to have a financial statement audit completed. I would be interested where you are getting your information that auditing a company is not a normal process.

Public companies have to undergo a financial audit every year. If you’re referring to a tax audit from the IRS, then yes, those are rarer.