Main reason is there is no way to select a specific IL without just having a save there. RE5 is great for ILs, since we can pick any segment and build an inventory for that segment right then and there. In fact, because the IGT resets with checkpoint reset in that game, we even have 2 sub categories of ILs for SS and segmented.

I guess you could do this in RE4, just by making the inventory you want for a specific IL and making a save, but that seems like a pain and there simply has never been any interest in it, especially when you could just do a segmented run if you wanted to optimize chapters. If sizable interest ever arose for ILs then yeah, we could definitely make it happen. It could even still be IGT by simply subtracting the time you started at from the time you finished according to the typewritter.

If it ever did become a thing though, I would advocate for it to be NG+ only, since there would be no way to gauge appropriate inventory for NG (this is how it is on RE5 currently, NG+ only)

Also if it happened it could be an alternate option to make it into the 5 full chapters (1-1 through 1-3, 2-1 through 2-3, etc.) I have seen some games that treat ILs this way