It makes more sense if you understand that the "thorn" (Þ) is pronounced "th".
Interestingly, the thorn was in pretty common use until the printing press took off because most of the presses in England were imported from France and Germany, neither of which used the thorn so their typefaces didn't include one. For a while people used 'y' in place of the thorn (hence "ye olde"), but eventually it fell out of use all together
Printing press is one factor, another is French influence. Greek terms with that sound were written with <th> like in French and so <th> already competed with <þ> independent of the printing press.
The thorn evolved as a pseudo glyph first, have you ever written a "th" really fast? Once the printing press was invented and widespread, it became less common for "th" to look like a thorn and it slowly fell out of use altogether
A frog is a wee beast with four legs which lives both in water and on land. He is brown, green, or yellow, or if he is tropical, he may be diverse colors. He has lungs and gills both. He haches from an egg and he then is a tadpole. He grows to be a frog if he is not eaten.
Actually early Middle English and Dutch were not that far apart. More French, of course, but a lot of Germanic verbs and vocabulary that matched up with Dutch.
I read this all in a broad Scots accent. Which is possibly a pretty accurate choice. Old English Early middle English and lowland Scots are very, very similar as languages.