The difference in context is what is important here. At the time of the battle of tuetoberger Wald where the Romans famously lost a number of legions, or even prior to this in the time of Caesars conquest of Gaul, the Germanic tribes (however poorly the word tribe fits) were small and fractious, covering a large area of uncleared forests. The labour required to transform this land into productive homesteads similar to Gaul would have been enormous.

After the Romans defeat at the hands of the Germans in the 1st century AD, Augustas effectively set the boundaries of Roman expansion. While they would continue to grow, with the aquisition of Britain in the 1st century, and those of Dacia and mesapotamia by trajan in the 2nd century, Hadrian rolled back much of these expansions and focused his energy on securing the defensible Roman boarders, running the length of the Danube, demarcating Rome from Germania.

The Marcomanic wars during the reign of Marcus Aurelius and the crisis of the 3rd century that followed set the end to any real possibility of Roman expansion into Germanic territory. This was a time that coupled Roman weakness (political instability, plague, a rising Persian empire) with an increase in the cooperation of the German tribes into larger confederations. These Germanic proto-states had the man power and the organization to be able to challenge Roman power on both sides of the Danube, and were a far cry from the disunified groups that inhabited the area in Caesars day.

From the time of Aurelian then, the Roman focus had shifted dramatically from an empire of conquest, to one which focussed on maintenance of it's territory. The 3rd century had shown that powerful men could not be trusted with large armies under their control, limiting Roman ability to campaign on multiple fronts. Persia was a constant threat, and Germanic raids/migrations/invasions rocked the western empire and it's ability to defend it's territory, let alone Rome's ability for conquest. The German tribes who would eventually inherit the western empire had become too strong and too unified for the empire to contain, leading to German expansion into Roman territory, rather than the other way around