Why didn't the Germanic tribes harry the Roman troops to distraction when Caesar built the bridges across the Rhine?

Question

The two bridges across the Rhine river are considered among the greatest military engineering achievements in the ancient world.

Caesar's Bridge across the Rhine, the first two bridges to cross the Rhine River on record, were built by Julius Caesar and his legionaries during the Gallic War in 55 BC and 53 BC. Strategically successful, they are also considered masterpieces of military engineering.1

What is not clear to me is why the tribes across the Rhine, who believed they could stop any crossing legions in boats by harrying them during their crossing, didn't employ the same tactics while the bridge was being built.

Answer 1

Of course no certain answer is possible, Caesar himself being the only (and quite non-independent) source

Nevertheless, the Imperator claims that the Germans were so frightened and awed by the ongoing construction, that they either sent envoys or fled:

Within ten days after the timber began to be collected, the whole work was completed, and the whole army led over. Caesar, leaving a strong guard at each end of the bridge, hastens into the territories of the Sigambri. In the mean time, embassadors from several nations come to him, whom, on their suing for peace and alliance, he answers in a courteous manner, and orders hostages to be brought to him. But the Sigambri, at the very time the bridge was begun to be built, made preparations for a flight (by the advice of such of the Tenchtheri and Usipetes as they had among them), and quitted their territories, and conveyed away all their possessions, and concealed themselves in deserts and woods. (De Bello Gallico, book 4, 18)

It seems unreasonable to us, but one can imagine that the tribes were impressed by such a work that they had always believed to be impossible. Indeed, in the previous section, Caesar names exactly this (his and the Roman People's dignity) as the motivation for the work. (Suggesting that he could have crossed by boat, but deemed it more majestic [and a bit safer] to build a bridge.)

EDIT: Some additional information extracted from the same source:

There were at least one tribe, the Ubii, who were friendly with Caesar and promised ships for the crossing, but Caesar decided not to rely on them.

Caesar still did consider the possibility of the Germans trying to destroy the bridge, and had precautions: There is no reference the defenses against shipborne firing assault (the bridge was probably built from the gallic side), but he deployed works upriver to ward of heavy objects that the enemy might release into the flow:

piles were driven into the water obliquely, at the lower side of the bridge, and these, serving as buttresses, and being connected with every portion of the work, sustained the force of the stream: and there were others also above the bridge, at a moderate distance; that if trunks of trees or vessels were floated down the river by the barbarians for the purpose of destroying the work, the violence of such things might be diminished by these defenses, and might not injure the bridge.

EDIT2: Regarding the second crossing.

At the second time there were even less resistance. The Ubii sent pleas explaining that they were innocent in the charges the proconsul launched the expedition for (having sent auxiliaries to the revolting Gauls), and laid the blame on the Suevi, who again collected the forces of their tenant-tribes (or whatever is the proper term), and retreated into the deep woods.

Answer 2

There are good reasons why the German tribes didn't worry a lot about a riverine assault but did worry about one over bridges:

Riverine and maritime assaults are extremely difficult affairs for the attackers. The odds favor the defender. It's not a coincidence marines and paras are amongst the elites of every armed force. Especially when defending their home lands.

Caesar had landed at least once in Britain, which was a botched operation at best. He know how difficult a naval landing would be.

Next, the difference in training, logistics, weapons and armor. The Romans had it, the Germans none of it. Supposing the Roman army would cross the river in barges and boats, they had a chance. Attacking over bridges: no chance at all. None whatsoever.

Third on the list is reputation. Caesar did have some setbacks, but at that moment no major defeats. That goes for his troops as well. The Germanic tribes would have to face a victorious (mostly) commander with (almost) invincible troops. In open battle. That's exactly in what the Roman excelled and what the Germanic tribes feared the most.

Now for logistics. The Romans could replace losses and send more troops if required. All they had to do was send a letter to Rome. I don't say that Caesar would get them, but his logistics were pretty much okay. The Germanic tribes fought as individual tribes, not under a united or unified command. Some tribes cooperated, most didn't.

Finally, the engineering feat. Building a bridge over the Rhine was for the Germanic tribes a feat of engineering they thought to be impossible. I'm sure they watched dumbfounded see it build, and build that fast, like we would look at a spaceship taking off. Seeing the impossible build within a fortnight does tend to take the fight out of anyone.