It is genetic. Prepositions in Indo-European languages come from adverb-like particles which themselves often come from some sort of noun in a specific case. This adverb could be mostly anywhere in the sentence given the relatively free word order of PIE.
In English, many of these adverbs never fully transitioned into pure prepositions as they can stand on their own:
The treasure is 2 meters below/above the surface. (preposition)
The treasure is somewhere below/above. (adverb)
I came through hell. (preposition)
I came through. (adverb)
Fire in the hole (preposition)
The spy is in. (adverb)
Furthemore, these adverb like particles not only became prepositions in the languages (i.e. noun modifiers), but they also became verbal prefixes. Again, in English the move to the verbal prefixes did not happen all too much (another example is German where there are still verbs with separable prefixes).
The possibilities of development can be observed in ancient Greek:
Oreos bainó kata. - mountain-GEN go-1SG down
Kata oreos bainó - Down mountain-GEN go-1SG
Oreos katabainó. - mountain-GEN down-go-1SG
In some languages, it can even combine the two, adverb became a verbal prefix, but the verb still requires a noun with the same preposition - e.g. in Czech you may have both:
Přejdu přes řeku - Over-go-1SG over river-ACC
Přejdu řeku - Over-go-1SG river-ACC
So basically, I believe it is safe to say that it is mostly due to chance that in Germanic languages the grammaticalisation went more slowly (to the of not being finished in English) than in other IE languages.