First of all, English is not a single language; it is a polyglot of French, Latin, Old English and other stuff from the British Isles. The original primitive English was much more regular in grammatical constructions. For example, originally the English said "I have" (first person) and "Thou hast" (second person). This grammatical structure gradually disappeared as English became a conglomeration of other languages. This same pattern is true of other polyglots. For example, Singlish, the polyglot spoken in Singapore, has very little grammar. Instead, like in English, function words are used to perform grammatical needs.
Also, languages appear to just degrade over time. For example, Greek is slightly less structured than it was in ancient times. It is not exactly clear why this has happened, but one possibility is just the wear and tear of the ages. Greek was under the control of conquerors like the Romans and Turks for hundreds of years. During those times, the Greek language went relatively untended, like a garden growing weeds.
Secondly, Latin is an outlier. It is possibly the most formal and structured language known and having had a wide use. The Romans did indeed fully use their language and greatly prided themselves on its proper use. Rhetoric and grammar were considered to be the most important parts of a young person's education. By the Lex Cincia it was actually illegal for people accused of crimes to hire a lawyer--they were expected to be able to plead their own case, pro se, (although advocates pro bono were allowed). The Romans considered themselves to be superior to everyone around them for three reasons: they shaved, wore the toga and they spoke a sophisticated language, but others had beards, breeches and crude, barbarous tongues. Thus, mastery of language was considered one of the essential elements of what it meant to be a Roman. In Roman plays, the parts of Romans always use proper language and foreigners are depicted using slang and vulgar, ungrammatical speech. The Romans considered proper and exact use of language to be the mark of cultural superiority.
It is interesting to compare Japanese to Latin. Like Latin, Japanese is a highly structured and formal language. However, unlike Latin it is intended to be simple, not sophisticated. The Japanese take pride in this, saying always "Japanese is simple." Of course, to learn it is not, but the constructions are intended to be simple and logical. The reason for this difference from Latin is that in Rome, rhetoric, the art of persuasion and communication was considered critical, but in Japan the reverse was true: people were expected to obey higher ups. Arguing in Japan is considered uncouth. Therefore, Japanese is the language of giving orders and social structure. There are elaborate forms of speech in Japan that carefully identify the social rank and position of the speakers. Latin by contrast is a language of social equality. Thus, you can see a language, including its complexity and sophistication, reflects its governing society like a mirror.