Well, when hiking through unfamiliar mountains, which I'll wager happens with a lot more than 3% of us, it's not always sure if what awaits is apples or oranges.
Sounds like we both spend a lot of time in familiar territory, so we both know enough about what to expect to be able to do without rain pants. But going into unfamiliar territory is exciting, and popular with many. The long stretches well above timberline on the Continental Divide in CO, for example, might well merit rainpants if they were in other regions where the rain gets long and merciless.
Not to mention the current changes in climate that are full of surprises.
And it sounds from this thread that even where rain pants are not critical, they can often make a trek much for comfortable. So while I don't use them, I might in different places, and so don't rule them out altogether.