That's because everybody who was anybody invaded Britain and liked it so much that they stayed long enough to inflict some of their language on the local people.
This included the Romans (Latin), various Viking groups (different Norse/Teutonic languages), Angles/Saxons/Jutes (ancient German/Teutonic ), the French (ancient French). Later on, as international trade progressed, this included Portugese, Dutch, Spanish, Indian, etc, etc, etc.
Along with the vocabulary, some snippets of grammar slipped in.
And then you've got the various dialects of English on the British mainland, starting from the Cornish in the south to the Scots in the north. And not forgetting the Welsh, and all the various flavours of London English.
In the 1700's some English academics tried to enforce some formal structure to English by adopting a whole heap of Latin grammar rules, like gerunds. This, however, did nothing for the various flavours of vocabulary and accent.
Nor did it prevent such abominations as the word 'hamburger', which, while including beef, lettuce, tomato, cheese, egg, bacon, pineapple and (canned) beetroot, would never include ham.
It is truly astonishing that any two British people can understand each other, much less foreigners understand them...