Or because it really was poor and inhospitable. Until the middle ages, when new agricultural tools were invented, Germanic territories projected very little power, and relied on periodic aggression to survive in typical nomadic way. This changed when they could actually grow stuff in more reliable ways, producing surpluses to trade and establishing stable kingdoms.
In other words: Germanic tribes were using the defensive strategy of not preparing wheat fields for invaders to harvest during campaigning season. Kind of equivalent to cyberdefense by pen and paper.
Half of Cesar's Gallic War seems to be more than half about grain logistics (huge efforts to start the campaigns jus a few weeks earlier) and ripeness of the local crops. And that is even while he was likely trying to downplay all the robbery.
Romans didn't just pillage, they also planted stuff like chestnut trees as they moved up and down Europe, so that they could use them in following years. Regardless, I find it difficult to believe that already-struggling tribes would further starve themselves just in case this or that opponent tried to invade. It's much more likely that they simply weren't able to grow much more in what was a hard soil, without the stronger plows that would be used later on to break such soil more effectively; especially when a lot of these people were still basically nomadic.
Production beyond the Rhine simply was not great even in times of peace, before the middle ages. Rome ended up controlling most of the German territory through client states anyway, even sending troops for punitive raids and propping up this or that friendly ruler, so they knew the economic potential of those lands pretty well; they just renounced full invasion because it was not worth the risk. At a time when they were already hitting what we could call scaling limits in their ability to mobilise troops over long distances, there was little appetite for going further North, where clearly there were no riches waiting for them. A similar assessment was done for Scotland, and rightly so.
> Regardless, I find it difficult to believe that already-struggling tribes would further starve themselves just in case this or that opponent tried to invade.
Allow me to apologize for my lack of precision, I never intend to imply that they deliberately avoided "invader-friendly" crops. Wheat just had not spread that far, due to climate (not yet sufficiently adapted by breeding?) and/or cultural reasons: nomadism (well, more effect than cause I guess) and the fact that large scale forest clearances are the type of project that can only happen in presence of big organizations that are stable enough to enable such long term investments. Many German settlements still carry the name of the medieval nobleman who commissioned the original clearance (names ending in -rode, -roda, -reuth and probably some more regional variations), which implies that before the clearance, there was only wilderness.
Not to mention the fact that by the third century, if a general controlled enough troops, they would (and did) overthrow the Emperor, so the Emperor tended to run around with one large army putting out fires, which was inefficient.
And many of those soldiers where mercenaries from the outside, who, by serving the empire, where put in the position to loot the empire's superior agriculture.
This went on for many generations, it's not a "last days before the ostrogoths" exception. Competing "emperor startups" in an empire representing pretty much the entire known civilized world, surrounded by mostly harmless outlaw wilderness. It must have felt as natural and "could the world even be not like this?" to the people populating that world as nation states, competing militarily, economically and in the Olympics seem natural to us.
