But for some reason, German nearly always comes out as laughable borderline gibberish. Part of it could just be that I'm mostly reading product reviews from "real people" which use everyday language and idioms more frequently, but even official copy from companies comes out at best hilarious and barely intelligible. Any clue why that is? I always thought of German as being incredibly close to English (relative to most other languages) but as you pointed out, the state of English/German computer translation is quite sad.
In German you can easily construct sentences which become gramatically very complex in comparison to English. Also, the syntax, i.e. order of words, is often more flexible and also plays a big part in the meaning of a sentence. I actually found a pretty neat example when I just googled the topic (link below):
Sources: Briefträger beißen Hunde selten. (Mind you, writing "Hunde beißen Briefträger selten." does not change the meaning.)
Translation 1: Dogs seldom bite postmen.
Translation 2: Postmen seldom bite dogs.
Both translations are correct, it is up to the reader to interpret the ambigious sentence by context and common sense, thus identifying whether the source is in Subject-Verb-Object or in Object-Verb-Subject word order. I just tested it and as expected, Google Translate and Bing Translator both are suggesting variant 2 as "Mailman rarely bite dogs", probably because Subject-Verb-Object is predominant in common speech. As you can see, they also fail to recognize "Briefträger" is plural. So I changed it to "Postmänner" and, tada: "Post men often biting dogs." - Now it's complete messed up.
Another oft quoted thing with German is that you can build arbitrarily long words of different nouns. (Infamous example: "Donaudampfschifffahrtskapitänsdienstmütze" meaning "hat of the captain of a steam ship on the Danube river which he is wearing on his duty")
So as you see, the way Google Translate and Bing Translator are designed, utilizing look up tables of man-made translations, will not work well in many cases and also give you some seriously flawed results.
As to why the combination of English and German is so bad in particular, I don't know - I am not an expert in machine translation, just your everyday user. But from my experience, there are other languages which also show how lacking these tools can be (thinking of Russian).