Because the Late Roman Empire had several economic, demographics and military problems. Plus the land of Germania was a wild and "poor" land for Roman standards, there wasn't anything to gain from conquering warmongering tribes on the other side of the Limes. Also the Romans thought of Germania as part of the empire actually, just unruled. At least de jure they considered the province of Germania their territory anyways

So the Romans did campaign along the Rhine for a very long time. You have Julian's Campaigns as Caesar against the Alemanni, and his successor Valentinian and Gratian campaigned along the Rhine.

The only problem was after that period, the Gothic War drew Roman military power towards the Balkans and the majority of Roman Gaul was left under the protection of the Franks.

However once the barbarian invasions of the early 400s happened. The empire in the West was too busy dealing with that to engage in campaigns into Germania.

The Romans retaliated with excursions well into the third century CE. There is a battlefield in lower Saxony that is dated to 235 CE, the Harzhorn battle. There is a museum there now with a very detailed description of this site, which seems to have been a major battle, not just a skirmish, between Romans and Germanic tribes. But the Roman empire at this point did not to have the resources to occupy and romanize Germania east of the Rhine.

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sources: https://roemerschlachtamharzhorn.de/

There are several factors involved including the effectiveness and numbers of the army, the health and numbers of the Roman population, the strength of the economy, the competence of the emperors, the internal struggles for power, the religious changes of the period, and the efficiency of the civil administration. All of this made the western half of the empire weak and vulnerable. They didn’t even deal with the non-Romans who crossed the Rhine, let alone those who remained on the other side.

Historians like Peter Heather (Empires and Barbarians) are good on this. For the first 250 years, Rome regularly extended its power across the Rhine and Danube. It negotiated treaties with the German and other groups, on free access for Roman traders (and slave-merchants), access to Roman goods and markets and so on. Bad behaviour was punished by legionary punitive expeditions, cutting off trade, diverting access and favoured treatment to rivals or just inviting the chief to dinner to discuss issues and then killing him. Rome levied drafts for the auxiliaries and took families in and distributed them to provinces short on labour. In other words, all the usual imperial playbook.

Over time the balance shifted. Germania developed, the tribes coalesced into bigger federations, the better to resist Roman pressure (the Alamanni, Franks, Goths, Marcomanni). Roman wars with these were tougher and more expensive, and paid less dividends. The treaties became less one-sided. As civil war, plague, agricultural exhaustion and so on weakened the Empire, the balance shifted. For the last hundred years or so, the Germans were trying to participate in Rome (often as defenders against other groups also wanting a piece), and Rome needed them too much to dictate terms. Then in 350-400 it fell apart under Hunnic and Gothic pressure.

They did, multiple times (Valentinian did one in the 360s, for example). However, Germania was too poor and backwards for a proper Roman occupation to be profitable (too many forests, too few proto-cities), and so the empire at first preferred to play the German tribes one against each other, with the occasional raids andpunitive expeditions. Later, things got out of hand, but at that point the empire couldn't have conquered the Rhine permanently even if it tried

They couldn't even fund themselves, not to mention a sufficiant army. I think if they could get an army off the ground, that could counterattack, wouldn't they have had a lot less problems? It's because they couldn't, that the empire disintegrated piece by piece.

One of the worst Roman military disasters was an expedition across the Rhine. A couple of legions got ambushed and wiped out at the Battle of the Teutoburg Forest (https://en.wikipedia.org/wiki/Battle_of_the_Teutoburg_Forest).

Cologne was the seat of the Agrippina family. Not far off was the site of Teuteborg Forrest 9BC and the Catalaunian Plains 451 AD. Germanicus moved his seat to Cologne. The family had be allies to the Sicambri for thousands of years, and the Sicambri held a bit of influence over the Belgae and the Germani.

I thought raids across the border were standard for pretty much any ancient empire or state. But eventually the western Roman Empire got invaded, with Goths etc entering en masse. Hard to raid Germania when you’ve been invaded because you ran out of soldiers.

I believe they did a few times i remember reading about it on Wikipedia. But it was considered to undeveloped and far away to be worth the time to occupy govern. Roman Influence was still strong through the presence of outposts and trade that they had considerable sway on the politics of the area though.

The Romans describe Germans as people who price keeping cattle over doing agriculture and have the annoying habit of burning down their own villages, granaries and fields and retreating into marshes or forests with their cattle if attacked. Feeding your army and ambushes will a big problem, and there is little loot to be expected.

Germans were poor and the land was seen as cold and worthless. Plus Germans were much taller (Romans were super insecure about this) and crazy warriors that weren’t worth the trouble to the short and “cultured” Mediterraneans. Funny how Italy is more German than Roman now huh. Apparently pants and lack of slavery is in vogue now

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