To sum it up: The costs simply outweighed the benefits.
You have to consider that Germania at this time was essentially one huge forest, which was very, well empty. No cities to conquer, the first German cities were actually founded by the Romans, like e.g. Aachen, Cologne or Trier. The Germans were primitive tribesmen and had little to offer to the Roman Empire. Yet they were warlike and fought many hard battles against them. Although the Roman armies were generally much more advanced with regard to arms technology and tactics, there were also huge setbacks like the batte Battle of the Teutoburg Forest.
Even Germanicus' campaign from 14 to 16 AD, is not considered a success. While Germanicus won the battles with only small losses, he lost ships and material to a storm in the North Sea after a generally successful campaign, and was recalled later.
Consider also that the northern european climate is not very attractive for people who are used to the mediterranean. You might want to read what Roman historian Tacitus wrote about Germania, the land and its inhabitants:
Then, besides the danger of a boisterous and unknown sea, who would
relinquish Asia, Africa, or Italy, for Germany, a land rude in its
surface, rigorous in its climate, cheerless to every beholder and
cultivator, except a native?
Another fact that should be taken into account is that the Roman invasion actually created a dangerous enemy for the Roman Empire, as the German tribes of that time were rather small groups that were hostile towards each other. In my opinion, only the threat of the Roman aggression allowed leaders like Arminius or Marbod to unite them into larger groups that presented a real threat at the Roman borders.
So that eventually, the emperor Tiberius recalled his nephew Germanicus and decided to leave the Germans to their own discord (I can't find an English translation of the exact quote). In my opinion, this is exactly what the Romans would repeat later in northern England/Caledonia, where they decided that further conquests of hostile territory and peoples were not worth the effort, and just pulled up a wall (the limes in case of Germania, Hadrian's wall in England) to guard the frontier.