They are written as though they rhyme, and Thou was the second person informal of middle English. The second person informal in French/Spanish/Italian/Portugues is Tu. The second person in German/Norwegian/Swedish/Danish is Du. They have the same vowel sound as You. So why does everyone pronounce "Thou" like "TH-OW"?
If I may add on, in Blake's poem The Tyger, he apparently rhymes "eye" and "symmetry". How were these words pronounced around the year 1800?
Not sure about the linguistics here, but from a poetry perspective "visual rhymes," where the words end the same visually but not audibly, was an accepted method of rhyming, at least in the 18th and 19th century. Many of Shakespeare's sonnets have rhymes like this and they have plenty of rhymes that are even less defensible to modern sensibilities than this.
The modern Icelandic word for you (singular) is "þu", which is descended from the Old Norse. It's like pronounced as if the word thou were to rhyme with the word you. I understand that the Old English as well as the Old Norse is also 'ðu' or 'þu' (both the ð and the þ symbols represent the 'th' sound). Both languages have a Germanic root, form the word 'tu'.
The French tu is pronounced differently. In phonetic it's [ty]. [Tu] means all. Source .I'm French