Unlike in Gaul, there really wasn't much to conquer in Germania. The Gauls had cities and farms and trade but the Germans were mostly nomadic. The man who conquered all of Gaul, Julius Caesar, describes the Germans like this:

"They do not practice agriculture, and the majority of their food consists of milk, cheese, and meat. No one possesses a fixed area of land or estates of his own: rather, every year the magistrates and leading citizens assign each family and clan who have joined forces a tract of land of and appropriate size and location. Then after a year they oblige these men to move on." (The Gallic War book 6.22)

Ariovistus, a leader of a German tribe who pushed into Gaul, boasted to Caesar that his men "who, with their extraordinary skill in weapons, had not been beneath a roof in fourteen years." (GW 1.36)

When he did cross the Rhine into Germany, Caesar was always concerned about the food supply for his army as the lack of agriculture meant there was nothing to pillage or demand in tribute from conquered tribes as there was in Gaul. And when he did find a settlement the Germans had already abandoned it and fled into the deep forest.

Ultimately, in both his expeditions into Germany he set out with a specific goal to punish German tribes who had attacked his Gallic allies and promptly crossed back into Gaul and burned the bridge behind him. He found it easier to build ships in hostile territory and make a contested landing in Britain than conquer any territory in Germany.

Later wars would push further into Britain but there as well they found it easy to conquer the settled lowland farmers than the semi-nomadic Picts in the highlands.