@J44xm
I don't know why they write ピィ instead of ピ ー.
First, do you agree that "dough" is pronounced as ドウ? (Some people have a hard time hearing it.)
OK. To Japanese people, saying ドウ is something they just do not worry about, or care about. It IS easier to say ドー, and that is what they say.
Another factor is the power of the written word. Many English words that contain a diphthong do not have the diphthong spelled out in the word, or the spelling is just downright bizarre. Take the English word "doe." It is exceedingly difficult for a Japanese person to look at this word and know how to pronounce it correctly. They simply take the consonant, take the vowel, they get ドー, and that is as far as they go.
You must also remember that hundreds of thousands of Japanese people only use English as a written language - they never use English listening or speaking skills. As such, they have no system at all to even let them know they need to worry about diphthongs.
Another factor is that non-diphthong spellings of English are very much standardized into Japanese. Hundreds of thousands of Japanese people will immediately say that the City of Oakland, when rendered as オウクランド is wrong and オークランド is right. A Japanese person who spells out diphthongs will stand out from the crowd, something very much discouraged in Japanese society.
Take something as simple as the city-name Tokyo. Japanese people say トーキョー while Americans トウキョウ.
Also, remember that diphthongs such as ドウ do not exist in Japanese. Getting Japanese people to use them just makes the whole job more difficult.