Halliday &
Matthiessen (2014: 424):
… prepositional Complements increasingly tend to have the same potential for becoming Subject, as in this floor shouldn’t be walked on for a few days. No doubt one reason for this tendency has been the lexical unity of phrasal verbs; because look up to is a single lexical item, with a one-word near-synonym admire, it is natural to parallel people have always looked up to her with she’s always been looked up to.