Word frequency is not a good statistical metric for a game where you care about letter frequency within words. If you use word frequency you end up drowning in H's and T's unless every other word you make is THE or contains TH. If you can track down a pre-V23 copy you can feel that first hand.
I is less common in shorter words than it is in longer words. Considering only words of 3-6 letters length it's around the 5th most common letter. Above that it's the most common letter by an increasing margin as the words get longer.
Making it less common on its own would make it harder to make 7+ letter words. Having the algorythm consider what's already on the board would be a possiblity, but that might inadvertedly nerf things like the Book of 4-heal.
As it currently stands for I, it's the third most common letter, which is a middle of the road compromise between short words and long words.