It's a very complex question with a lot of different causes. Early on, from around ~100BC the Germans were very small tribes without big population centers. There was a lot less food going round, given that most agricultural techniques in the area originated in the fertile areas of the Mediterranean, and were not as useful in the tough soil of Germany. Lack of population centers meant it wasn't as easy to gather them together and conquer them, as Caesar did to the Gaulish tribes.

These amongst other factors led to Rome having difficulty making inroads into pacifying Germany. Augustus decided after the disaster at the Teutoberg forest, that it was time to consolidate borders and Germany wasn't worth the trouble.

What a lot of people don't realise is that by virtue of being a nation on the border of Rome, a vast amount of Roman military knowledge, technology and manpower gradually diffused across the border. This effect was exacerbated by events like Domitian paying off Decebalus the Dacian chief to keep the peace with a lot of Roman technology and weaponry. Over time, there was also a significant contingent of Germanic auxiliaries serving in the Roman army.

The net effect of all this was, a huge load of tiny warlike tribes that were too difficult to pacify and thus left alone, gradually over the decades/centuries absorbing and incorporating the techniques and technology of Rome, eventually consolidating into bigger and bigger tribes (due to various societal reasons). As you can see, this is a recipe for a pretty big threat