Technically, they did, and they were quite close in the 3rd century. However before then it might've been more debatable.

Peter Heather talks about this a lot in his book "Empires and Barbarians", but I've seen it mentioned elsewhere in other books. The problem is that Germanic tribal political structure at the beginning of the empire was worlds different from that of the late empire. It was looser, more temporary, more dependent upon military success, and less inheritable from a broader coalition standpoint. The preference was not for large permanent confederations or states but smaller tribal groups.

The germans essentially didn't NEED to have such a permanent unified force to permanently go toe to toe against the empire in the 1st century as they did in the 3rd century-5th century.

This is kind of exemplified by Arminius' victory over the Roman legions at the Teutoberg forest in 9 CE. After the victory, he hoped he could be the person who united the disparate german tribes as a force against Rome. Instead people in his tribe decided to kill him when they feared he would get too powerful.

Keep in mind too, at the time of Teutoberg Forest, direct Roman military interaction with the Germans was probably only 70 years old, coming off the back of Rome's gallic conquest. Compare that to the time of the Marcomannic war in the 2nd century, by which the Germans and the Romans would've been in close contact along the frontiers for 200+ years, and the 3rd century crisis, which would've been 300+ years.

I'm sure you'll notice, that I'm only saying that German political structure coalesced as to be able to challenge Rome more directly, though I'm not saying why. Unfortunately, the why here is much debated. Some historians say it was a direct reaction to the military pressure put on by Rome, others say it was an internal cultural reaction to the military rewards offered by Rome. Whatever the case, there is little doubt that interaction with Rome was an essential part of the equation.

It's just that it took a certain amount of time to change the political culture to be one that could not only defeat a Roman army in battle, but offer itself as a credible alternative to the Roman state.

Which is REALLY what happened in the 5th century. The barbarians didn't invade and immediately takeover. They invaded, they sometimes won sometimes lost, were settled, and as the Roman state collapsed (whether internally, or because of the barbarians, or both) became the better governing alternative.

Though the last precedents weren't set in the 3rd century crisis invasions, I could see a contingency where a few things gone more wrong with the empire, the 5th century process unfolding in the 3rd. Which would give you a potential scenario for the Germanic tribes "defeating" the Roman Empire sooner.