I had never actually thought about this before now, probably because I’m a native speaker of English. But once I gave it some thought, I was actually a little surprised that adjectives in English do not have agreements (as far as I know), especially given the numerous intricacies of the English language and its largely French/Latin influences.
Did English have adjective agreements at some point? Are there no adjective agreements because there is no grammatical gender?
According to the following source, it is not clear why gender agreement (which existed in OE) disappeared. It was probably due to simplify communication between Old English and Old Norse:
If you speak another language like Spanish or German, you are familiar with grammatical gender. In Romance languages (and many others), nouns have a gender. In French, a chair is la chaise, a feminine noun, and a hat is le chapeau, a masculine noun. But did you know that English used to have gendered nouns too?
Until the 1200s, English had grammatical gender. Instead of using the articles “the” or “a”, Old English had a masculine article “se” and a feminine article “seo”. The sun, for instance, was feminine, so it would be written “sēo sunne”. If you referred to the sun, you would even say “she”.
However, in northern England in the 1100s, grammatical gender disappeared. Historical linguists aren’t entirely sure why this happened, but Professor Anne Curzan suggests that genders were lost because of the language mixing that went on in Northern England during that time.
Between the 700s and the 1000s, there were Vikings invading northern England where peasants lived. The two groups spoke different languages: Old English and Old Norse. However, it is quite likely that many people were bilingual and fluent in both languages. Both Old English and Old Norse had gender, but sometimes their genders contradicted each other. In order to simplify communication, gendered nouns simply disappeared.
According to the following extract from Quora, the introduction, and usage of the gender-neutral article “þe (the)” is probably responsible for the decline in usage of gender agreement in the English language. I personally think this is an interesting assumption so I add it as additional information on this topic:
Unlike most other Indo-European languages which have gender, English introduced an article — þe (the) — that was gender neutral. Since most of England in the 13th-17th centuries was uneducated, it was a great time for language change. The article þe was much easier to learn than arbitrary gendered articles, so it started to be used by everyone.
And around the 14th century, Colloquial English started to drop the declensions present in proper Old English, and with it, grammatical gender disappeared.