Wild garlic (Allium ursinum, German: Bärlauch) is a native plant and not invasive in Germany. It’s not like it’s everywhere as it usually needs alluvial, deciduous forests but common enough that most locals usually know a spot or two. It may look like lily of the valley (Convallaria majalis, German: Maiglöckchen) or autumn crocus (Colchicum autumnale, German: Herbstzeitlose) which are both poisonous but that’s not the reason why you’re not allowed to pick large amounts. The Federal Nature Protection Law (Bundesnaturschutzgesetz, see paragraph 39 for details) forbids the collection of any wild plants or animals, independent of their protection status to disturb natural processes as little as possible. However certain exemptions are possible, think hunting, fishing, or the collection of wild herbs for example. While the former two require a license and are restricted to certain area or seasons, the latter is limited by the amount you may take. It explicitly states to only take amounts that are for personal use and that commercial collection is prohibited. It’s obviously to prevent overexploitation. Unfortunately, it’s not uncommon that naturally occurring stocks of wild herbs, mushrooms or berries are used up and destroyed by illegal and unsustainable usage. Also, yes wild herbs are commercially grown on farms but as you can imagine, they don’t come cheap at all.

Some species don’t grow very well in agricultural settings – especially forest products.

If you harvest too much from public property, then you’re breaking the law.

Maybe people will figure out how to farm it, which would probably lower the price enough so that people stop poaching it.