That is because the specific model of classifying a worker as a "self-employed contractor" when they work exclusively for a single company is not legal. Aka the gig-economy model used in other countries.

The governments view is that if you have fewer than 3 companies you are dependent on them and they should bare the AHV etc requirements. However if for example you have 2 very large contracts there may be exceptions.

Easiest (not cheapest) is always to create a GmbH or AG which also mitigates a lot of risk you would have as a sole-proprietor.