Anybody knows why The Canterbury Tales by Chaucer is considered as a documentary for the life in England in the 14th century?
No cameras, no videos then. It was just a vivid LITERARY depiction of the life then. True or untrue, a matter for dispute.
Geoffrey Chaucer - born in London in 1343, died in 1400 and is buried at Poet's Corner in Westminster Abbey, in London. Fluent in French and Latin, French of course being the official Language of the Court and Nobility of England, as it was, at that time. The link below gives you a reasonably good idea of his writings in early English, much of which is recognisable today to those of us who have English as our mother tongue.
The Canterbury Tales (unfinished, by the way) immediately comes to mind whenever his name is mentioned, and this gives us a good idea of contemporary life in that part of England, and is connected with the journey undertaken by the pilgrims from the area around Winchester, in Hampshire) across country to Canterbury, in Kent, and to the great cathedral there and pay homage at the shrine there to St Thomas a Becket, the archbishop who was rashly assassinated by four knights on the orders of England's King Henry II on 29 December 1170, after the King had cried out in frustration: "Who will rid me of this turbulent priest?", the culmination of a very stormy relationship between the monarch and the archbishop.
Chaucer's English -
The trail of the Pilgrims Way (from Winchester to Canterbury) is still partly visible today, and it's possible to walk along the whole trek from one great cathedral to another, across the truly beautiful Downs of Sussex and Kent, and, surprise, surprise, called The Pilgrims Way, and what no longer exists as the original Way has been taken over by the North Downs Way, and carefully preserved by various preservation societies, such as the National Trust and English Heritage.