I suspect that "The thing is" has become, as it were, "lexicalised".    From Wordnik.com.  [languagehat.com] Reference

Thanks Luiz – and, yes, most grammars create an ‘arch-category’ called multi-word verbs, within which both phrasal and prepositional verbs not to mention phrasal-prepositional verbs, of the type: look forward to, get along with… are sub-categorised, along with other anomolous constructions, such as “to make do” and de-lexicalised verb combos, as in ‘take care of’, ‘make fun of’ etc.    From Wordnik.com.  [P is for Phrasal Verb « An A-Z of ELT] Reference

Thanks Luiz – and, yes, most grammars create an ‘arch-category’ called multi-word verbs, within which both phrasal and prepositional verbs (not to mention phrasal-prepositional verbs, of the type: look forward to, get along with …) are sub-categorised, along with other anomolous constructions, such as “to make do” and de-lexicalised verb combos, as in ‘take care of’, ‘make fun of’ etc.    From Wordnik.com.  [P is for Phrasal Verb « An A-Z of ELT] Reference