Certainly there are languages more closely related to English, like Dutch or Swedish / Norwegian, that stand sort of midway between English and (High) German. But if we take older languages, like classic Latin, or those with many archaic features, like Lithuanian, the distance from English syntax and grammar becomes pretty large.
I suspect that languages tend to simplify in grammar in areas where several different languages have to coexist: say, England had Germanic, Celtic, and French languages interacting for quite some time, with some Latin thrown in by the church.
I suspect that Mandarin Chinese also resulted from many languages on a pretty large territory interacting a lot, shaving grammatical complexity bit by bit.
English's grammar didn't simplify if you mean its syntax by that, its morphology did. (Inflections affect morphology [word form] more than syntax.)
English's grammar actually got far more complex because word order matters a whole lot more syntactically versus German or French or Latin. For one big instance that is often pointed out by English as a Second Language problems, adjective order in English is comparatively extremely rigid: https://en.wikipedia.org/wiki/Adjective#Order
Another place you see this complexity is the old "sentences should not end in a preposition" early schooling "rule": in Latin word order mostly doesn't matter as long as key things agree on grammatical case/gender. Prepositions "can't" syntactically exist in Latin without their paired nouns inflected to the right grammar case. But the pair itself can be in almost any order in the sentence. Whereas English has a more rigid prepositional word order, because nouns don't inflect at all when used in a preposition (unless you are someone still trying to keep "whom" on life support centuries after it died in English and became a grammatical zombie). The syntax is more complicated, but the morphology is easier. One of the interesting "gains" from the complicated syntax is that some prepositions started working with intentionally dropped or elided direct nouns. That is also added syntactical complexity. In English there are many prepositions it is perfectly valid to end a sentence with. Parsing that is interestingly complex.
Extremely relatedly the similar childhood "rule" to "never split an infinitive". In many languages infinitives are a pure grammatical case involving word inflections that have to match, and the order generally doesn't matter if they involve multiple participles, and there is no such thing syntactically as a "split infinitive", it just can't syntactically exist. Whereas English relies on a more complex syntax for infinitives that relies more explicitly on rigid word orders, making split infinitives at all possible in the first place. Then regular usage of the different concurrent syntaxes started to diverge their meaning. "To boldly go" means something subtly different in English than "to go boldly", and forbidding split infinitives entirely forbids entire categories of creative expression in the language.
I don't think there's a simple answer for why English grammar evolved the way that it did in shifting morphological complexity for syntactical complexity. I just know that calling that evolution "simplifying" isn't entirely accurate either. (English syntax feels simpler to native speakers than it actually is.) The language certainly had some interesting linguistic pressures being confined to a highly regarded/desirable set of isles for centuries with a mixture of strong related languages immediately nearby, then when it was used as a language for helping bootstrap an industrial revolution and as a world-spanning empire.
Certainly there are languages more closely related to English, like Dutch or Swedish / Norwegian, that stand sort of midway between English and (High) German. But if we take older languages, like classic Latin, or those with many archaic features, like Lithuanian, the distance from English syntax and grammar becomes pretty large.
I suspect that languages tend to simplify in grammar in areas where several different languages have to coexist: say, England had Germanic, Celtic, and French languages interacting for quite some time, with some Latin thrown in by the church.
I suspect that Mandarin Chinese also resulted from many languages on a pretty large territory interacting a lot, shaving grammatical complexity bit by bit.