So the Romans did campaign along the Rhine for a very long time. You have Julian's Campaigns as Caesar against the Alemanni, and his successor Valentinian and Gratian campaigned along the Rhine.

The only problem was after that period, the Gothic War drew Roman military power towards the Balkans and the majority of Roman Gaul was left under the protection of the Franks.

However once the barbarian invasions of the early 400s happened. The empire in the West was too busy dealing with that to engage in campaigns into Germania.