One reason the Romans did not conquer Germany is that the area did not have enough resources worth the effort to steal.
They had better things to do, such as chop each other up in attempts to seize power. Civil wars disrupted Roman politics more than barbarian invasions — until there were no more Roman politics per se, and the barbarians finally took over Italy. But every region had resources, and in the negative sense, pushing borders further into unknown territory was a common practice by the Romans. They used bribery and trade to bring neighboring tribes into the Roman economy bit by bit and build proxy states as buffers, then later would annex these Romanized areas, and find the next group of proxies had already begun the process of integration on their own.