While a preposition establishes a relation between objects the conjunction establishes a relation between phrases or parts of phrases.

We do have two phrases:

He buys a car. He knows how to drive.

We can combine these phrases in different manners:

He buys a car because he knows how to drive. He buys a car although he does not know how to drive.

It's clear that those two phrases do not mean the same. The relation established by the conjunction because is quite different to the one established by although. The meaning changes totally using the one or the other.