The words hint, lint, mint, tint are all pronunced with a short-i sound. The vowel in pint is a diphthong however. Why ?
Unfortunately, I have found no reference that explains this, so what follows is my own speculation and might be totally off-base. I think this has to do with the etymological origin of these words. The words hint, lint, and mint all come from Old English. The word tint is apparently from Latin, with influences from Italian.
The word pint comes from Old/Middle French pinte (the sources I've looked at differ in the terminology). It seems that in many cases, vowels in Middle French words were taken into Middle English as long vowels even when they were followed by consonant clusters which would normally indicate a short pronunciation. I wrote an answer about the pronunciation of tomb that I think is relevant: Why do “bomb” and “tomb” have different pronunciations?
I know of no other words from French (or actually, of any origin) that rhyme with pint, but you can see this tendency to use a long value before nt in words with other vowels that were borrowed from Old or Middle French:
- -ount: mount, count, fount(ain) from Old/Middle Fr. monter, conter, font(aine)
(although on the other hand, we do have front with a short vowel from Old French front, and font with a different short vowel as a more recent borrowing from French)
- -aunt: haunt, taunt, daunt, avaunt from Old/Middle Fr. hanter, tanter or tant, danter, avant
(although on the other hand, we do have grant with a short vowel, and aunt which may have a short vowel depending on your accent)
You can see that there are a number of counter-examples where the vowel is pronounced short in Modern English. There is even one spelled with -int: print, from French preinte/prente. This weakens this explanation, but it still seems the most likely reason to me.
As for why we still spell it with i... as David Garner mentions in a comment, one of the oddities of the modern English spelling system is that there is no commonly used digraph that represents the diphthong /aɪ/ ("long i"). Words like feast and roast (from French feste and rostir) ended up being spelled with the digraphs "ea" and "oa" that represented "long e" and "long o" respectively. But there isn't any clear way in the modern English spelling system to spell "pint" to indicate the length of the vowel.
- Peint would be just as ambiguous and irregular (it looks like it should rhyme with feint)
- Pighnt would be less ambiguous, but it doesn't fit in with the normal distribution of the trigraph igh, which usually only occurs at the end of a syllable or before the letter t. The trigraph igh is also rare; as the spelling suggests, it originally represented a vowel-consonant sequence rather than a pure vowel, and this is its etymological origin in most modern English words that contain "igh." There are a few words where it is used unetymologically, such as delight (from Old French delit), but this arose from the analogy of all the other words ending in -ight; there are no existing words ending in -ighnt that could provide a basis for using this spelling for /aɪnt/.
The same ambiguity applies to i in other words, such Christ (doesn't rhyme with mist) and all the words ending in -ind that don't rhyme with the noun wind.