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.