By request, I'll unpack what I said in the comment above.
Part of the Great Vowel Shift and its aftershocks
which occurred between about 1400 and 1700, depending on where (and what variety of) English was being spoken. These things take a loooong time.
was a general tendency to distinguish vowels by quality (i.e, tongue position)
Like the vowels in beat and bit are different in quality, and are also different from the vowels in bait, bet, bat, bot, bought, boat, but, boot, and foot. They're all distinguished by where in the mouth the tongue is; that's vowel quality.
instead of length (i.e, the time the vowel was held).
Old and Middle English had, like German and French, long and short vowels. The terms "long vowel" and "short vowel" are often used in grammar school to teach English reading, but they're not really long or short.
If you're a native English speaker, you were probably taught that there were 5 vowels, and they came in long and short varieties. This theory describes Middle English, which did have a, e, i, o, and u occurring both short and long (i.e, held twice as long as short), though Middle English also had a long and short æ vowel that got lost during the GVS. However, it does not describe Modern English, which has no distinctive vowel length, but does have around 13 qualitatively distinct vowels. Middle English was the language that English spelling was designed for, so this explains a great deal.
The GVS changed the quality of ME long vowels, but the perception of length persisted in some dialects by diphthongizing the moved "long" vowels, now phonemic tense /i, e, o, u/, resulting in [iy, ey, ow, uw] as normal allophones of the tense vowels.
[w] is the labializing offglide for tense rounded vowels /o/ and /u/ (also for back vowels, because English back vowels are rounded, whereas front vowels aren't); and [y] is the palatalizing offglide for tense front vowels /e/ and /i/. These offglides affect what comes after the vowels.
This can be perceived easily by observing the behavior of tense vowels before resonants, especially /l/ (/r/ has other issues after vowels). Many English speakers use an epenthetic schwa in words like fool, feel, foal, fail.
I.e, in these words, for these speakers, there are two syllables -- the first stressed syllable with a tense vowel -- and the second, short syllable, consisting of a reduced vowel ([ə] or [ɨ]) and the final /l/. In essence, a syllabic resonant.
- In fail and feel, there's a /y/ between the two syllables: ['fiyɨl] and ['feyɨl]
- In fool and foal, there's a /w/: ['fuwɨl] and ['fowɨl].
Indeed, the phenomenon of tense vowel diphthongs is one of the major features of an English accent in other languages. Any native English speaker who can learn to say pure tense vowels [i:, e:, o:, u:] at normal speech rates, without diphthongizing their vowels, can improve their accent greatly in any European language, for instance, because none of them have this feature, but rather use pure vowels throughout, except for phonemic diphthongs like /ay/ and /oy/. And /ow/ and /ey/ are likely to occur and be phonemically distinct from /o/ and /e/ in those languages, anyway.
