Why can't English speakers understand German or German speakers understand English?

The languages look and sound similar with the exception of umlauts in German versus English. Words are spelled differently and past tense usually begins with ge and ends in t as in gehilfte. Ich gehilfte eine Freund and there are a lot of cognates but most words are totally different. So why can't either or of the speakers understand each other? This is weird and I don't get that like I understand both English and German and speak them fluently.