The US system's structure penalizes companies for hiring workers between 20-40 hours. Any company that doesn't rely on flexible <20 hour employees or salary + forced-overtime is basically throwing money away.
Tax code relating to benefits and labor laws (which kind of assume the environment encouraged by the tax code.)
As an example, benefits have fairness tests between highest and lowest paid employees. Fail those tests and the benefits are taxed, possibly as normal income. Companies find tricks to get people out of the group of normal employees under the IRS rules to beat these tests.
edit oops- <20..