$1000 is the purchasing limit in most large enterprises - anything over $1000 needs specific authorization. Buying a product at $999 is completely frictionless, while buying a product for $1001 involves multiple meetings and reams of paperwork. In enterprise purchasing logic, $999 is indistinguishable from free and $1001 might as well be $50,000.
Clever software marketers have realized that a $999/mo SaaS product fits under that limit just as well as a $999 boxed product.
Clever software marketers have realized that a $999/mo SaaS product fits under that limit just as well as a $999 boxed product.