I have noticed that many idioms in English include a fixed preposition at the same time that the complement of the preposition is free, e.g.
a. light a fire under X
b. carry a torch for X
c. cast a pall on X
d. keep in touch with X
The verb-preposition combination is fixed in these cases, whereas the complement of the preposition is free (as indicated by the use of X). One can vary the complement, e.g. light a fire under John/you/the students/ etc. In contrast, there seem to be no idioms in which the verb and complement noun of the preposition are fixed at the same time that the preposition itself is free, light a fire above/beside/behind John -- only the literal readings possible here.
Why is the preposition so special? What does this situation reveal about idiosyncratic verb-preposition combinations?