The USA has no equivalent of Employers NI (they’re not that daft) and ER NI is the biggest factor in the UK in any IR35 case. The differential between perm and contract in the US is much less in tax terms.

The remuneration paid is often similar as well, so there’s no financial incentive – contractors are often simply people who’ve not been able to persuade the company to take them onto the permanent staff, many of them aspire to have that happen.