The Germanic tribes had nearly identical cultures and language but they seemed to remain tribes and never formed any sort of kingdom until after Rome fell and they land grabbed.
Why did they never confederate into one massive tribe?
Why did no one tribe ever manage to conquer and absorb the rest and build a kingdom/empire in Germania?
The Germanic tribes seemed to have lived the same type of life style for hundreds or even thousands of years before Rome fell.
After Rome fell and the Germanic tribes land grabbed, why did they then decide to settle and build up rather than remain tribes like they had always been?
To answer your first question, German tribes did confederate from time to time. The Varian Disaster of 9 A.D., in which three whole legions under the command of P. Quinctilius Varus were destroyed by a German force under the command of the general Arminius, is one such case. Varus was given control of Roman territory to the Elbe River, and had provoked "the tribes in the vicinity to oppose Rome in closer co-operation," which suggests the creation of a larger entity.^1
One reason why a cohesive kingdom never emerged was simply the geography of the region. As Tacitus writes, "the country in general... either bristles with woods or festers with swamps," which is also part of the reason why the Romans could never truly conquer Germania; it was untamable, and the resources required to fully conquer it would have significantly drained the Roman empire of fighting men and finances that could be more wisely used elsewhere.^2
By the end of the Roman empire, the army was relying on increasing numbers of Germanic peoples to fill the ranks. By training the Germans in military discipline, organization, and structure, the Romans helped transmit the knowledge required to form more organized forces. Centuries of contact with Roman civilization as well, through trade, diplomacy, and warfare, had also given the Germanic peoples the basic foundations of a civilized (as opposed to tribal) society.
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