Why is "service" pronounced the way it is?

Why is service pronounced the way it is and not like device even though the last 4 letters of the words are identical?

I would think that if they end the same way, the same pronunciation rules should apply as well.

I also checked Wiktionary.org and it says both words evolved from Old French where 'service' originated from 'servis' and 'device' originated from 'devis'. Again identical end of the word spelling.

I believe this is due to the Great Vowel Shift.

In Middle English the "i" in device used to be prounounced as in service but later shifted to a /ai/ diphtong because it was bearing the stress.

It's entirely a matter of syllable stress. English is a stress-timed language. Only a stressed syllable can have a full vowel in English; unstressed syllables are centralized and reduced as much as possible, especially in rapid speech.

SERvice is stressed on the first syllable, so the vowel in the first syllable gets fully pronounced ['sɝ], while the second unstressed syllable is reduced to shwa [vəs].

On the hand, deVICE is stressed on the second syllable, so the vowel in that syllable gets fully pronounced ['vəys], while the first unstressed syllable is reduced to shwa [də].

Spelling has very little to do with pronunciation in English; its spelling was invented for a different language and doesn't work at all well for modern English. This fact explains a lot of other things, including why you shouldn't be worried if it's not logical. In fact, you're right. -- it's not logical.

The pronunciation of device might actually be considered less predictable than the pronunciation of service (if we compare them to words like novice and crevice). But then again, in the word advice we have the same pronunciation as in device. The stress pattern of device and advice may be related to their morphology: in both words, the first syllable was originally a prefix. It may also be related to the existence of the verbs devise and advise; if so, another similar example may be abuse n. and abuse v.

I'm going to ignore the presence of the letter "v", since I don't think it adds much information. So, here are some overall tendencies I see for the pronunciation of words that end with -ice.

Monosyllabic words ending in -ice all have /aɪs/ (unless the i is part of a vowel digraph, as in voice, juice, sluice). Some examples: ice, price, spice, slice.

Polysyllabic words derived from these also have /aɪs/, such as de-ice, re-price, overspice.

Most other polysyllabic words ending in -ice have fully unstressed /ɪs/ or /əs/. Of the words pronounced like this, most are nouns. There are some verbs, such as notice and apprentice, but they all have corresponding nouns spelled and pronounced the same way. In U.S. English, the verb practice also falls in this category. In British English the verb is spelled practise, although standardly pronounced the same as the noun. The OED has an interesting note about practise:

The word was originally stressed on the second syllable [...] and this is still the case in some regional varieties, especially in Scots (hence such spellings as practize, practeeze, practeese). The stress was subsequently shifted to the first syllable, with devoicing of the final consonant, probably by association with practice n.

Some examples, with the stressed vowel in bold: crevice, justice, lattice, notice, novice, artifice, avarice, edifice, orifice, accomplice, apprentice, interstice.

A small number of polysyllabic words have /aɪs/; as mentioned in other answers, these words always have primary or tertiary stress on the last syllable (and none of them have primary stress on the second-to-last syllable). This category has a greater proportion of verbs. (The reason why I've discussed part of speech is that in English verbs are generally more likely than nouns to have some degree of stress on the final syllable; we can see this in contrasts like associate n. versus associate v., or the noun ending -ity versus the verb ending -ify.) As StoneyB mentions in a comment, the stress may also be affected when the first syllable of a word comes from a Latin prefix. Here is a list of all the words like this I found, with the primary-stressed syllable in bold:

• advice n. (pairs with advise v.; from Latin past participle vīsum with the preposition/prefix a(d-) )
• device n. (pairs with devise v.; apparently ultimately from Latin dīvīs-/dīvidere, from videre with the prefix dis-)
• entice v. (from Vulgar Latin intitiāre, with the prefix in-)
• suffice v. (from Latin sufficere, with the prefix sub-)
• sacrifice n., v. (from French, ultimately from Latin sacrificium. The OED says the verb was derived from the noun)
• satisfice v. (note: this word was coined by combining satisfy and suffice)
• cockatrice n. (however, this word is also pronounced with /ɪs/)

There are also a couple of words spelled with -ice that are usually pronounced with stressed /iːs/: police, caprice. They were borrowed relatively recently from French, and don't show the signs of the Great Vowel shift.