Pronunciation differences between "finite" and "infinite"

I believe we pronounce infinite the way we do because a dactyl is simply easier to say, in most cases. It trips (quite literally) off the tongue. It seems we're forced into the spondee of finite, even though that is harder to say, because nothing else makes sense. But once that extra syllable has come to the rescue we can get lazy again.

Edit

After giving the matter more thought, I think we pronounce finite as a spondee because both syllables have the "long i" sound (aɪ). I can't think of a single instance of aɪ coming on an unstressed syllable. I think that's mainly true of all the "long" vowels as well, but I will not be surprised if someone can come up with an exception or three.

It's the three syllable rule that messes it up. Logically FI-nite would be in-FI-nite with the prefix, but as it becomes three syllables, the third syllable from the end is stressed and it becomes IN-fi-nite. When you stress the first syllable, you have to change the pronunciation to un-stress the second syllable.