Defenses along the German border predate Hadrian. However it is important to understand that these fortifications were never meant to be a barrier behind which Rome would fight off waves of barbarian attacks. Instead defenses were meant to be staging points for the projection of Roman force.

A display of such force would help dissuade attacks from beyond the frontiers. The mere visible presence of a wall, watchtower, or fort could achieve this end. Such structures would also be used to coordinate Roman action beyond the frontier. Punitive missions and patrols across the borders were equally important in keeping the tribes in check. We can characterize border relations as some sort of psychological game where Rome constantly tried to prevent attacks by appearing strong.

Rome however was not always strong and sometimes discontent amongst the tribes was strong enough to prompt raids along the defenses. If these went unchecked then larger raids would follow as the attackers were emboldened. We often see heavy campaigning in Germania by emperors as a way to return stability to the region.

In the face of heavy opposition, there was no real way lines of defensive emplacements would stop them. Attackers would merely go around strong points and break through where it was weakest. Garissoned troops would be more flexible than walls when it came to beating back an incursion. Along the huge perimeter of the Rhine and Danube the Romans therefore stationed large numbers of troops in legionary forts or later border guard postings.

TLDR: fear tactics and troops were more flexible and effective at guarding Rome's borders