The Roman Empire was huge. East Africa, Levant, Greece and others, all had been conquered by the mighty legions. But they never advanced beyond the Rhine, as if they had neglected these lands. In fact, the point here is not neglect at all. There are several good reasons why Rome was not too eager to expand its possessions in this direction.
It is worth starting with the fact that the Romans not only did try to take seriously the Germans, but also tried to defend themselves against them. Hadrian’s Wall in Britannia defended against the tribes there who attacked the legionnaires. A similar system of fortifications was created for the European land. This chain of fortifications was named Limes Germanicus, which has been partially preserved to this day. Fortifications were located between the Danube and the Rhine, on the very border of the Empire.
What could make the Romans, who were notoriously aggressive, so reluctant? The Germanic tribes were few in number and constantly quarreling among themselves, so why did hesitate? The problem was that after several clashes Rome had turned into a common enemy to all these tribes. Divide et impera, divide and conquer, did not work here. Basically, the threat of Roman aggression allowed leaders such as Arminius and Maroboduus to unite hostile tribes into larger groups that posed a real threat on the borders of Rome. While the Roman armies possessed undeniable advantages in terms of weapon technology and tactics, there were occasionally huge setbacks, such as the famous battle of the Teutoburg forest.
Another reason lied on the fact that to the Romans, Germanic tribal lands did not offer any significant benefits in economic terms. It was merely huge, empty forests. There were no cities to conquer. In fact, the first German cities were founded by the Romans, such as Aachen,and Trier. Besides, the Romans, who were accustomed to the warmer climate of the Mediterranean, did not find the Northern European climate attractive.
In the end, the conquest of Germania was simply not profitable enough. The costs outweighed the benefits. Make no mistake, if Rome took on the task of taking Germania with full force, they would surely succeed. But winning is one thing, economic interest is another. You need to gain more from the conquest, than what the conquest is worth.