If WHIP is a measure of a pitchers Hits and Walks per 9 innings, shouldn't a 'hit batter' be part of the equation? Isn't a 'hit batter' the same as a walk?
One of the appeals of using WHIP as a statistic is that it's very simple to understand and calculate, and the resulting numbers hold some real value as far as describing a pitcher's skill. If you were to add something to the formula (namely HBP numbers), you'd want to first look at how it affects the balance between 1. ease of use and 2. value of adding the new part.
In this case, we already have a statistic that can be easily understood and calculated by any average fan. Yes, an average fan should be able to handle adding in the HBP but is that small burden worth it? Just choosing a random league and year, I looked at the National League in 2016 and Jimmy Nelson led the league by hitting 17 batters. His actual WHIP of 1.517 would have been 1.612 if you include HBPs--a difference of about 1/10 of a point. Keep in mind that that's the biggest effect you would have seen for that year in that league. Also, practically every pitcher would have a slightly higher WHIP, just not by 1/10 of a point. For a simple stat like WHIP is intended to be, there's probably not enough value in adding the HBP numbers to be worth the trouble.
On the other hand, you're right that HBP is often a controllable thing and there are plenty of advanced stats that do take it into account, for example Field Independent Pitching. These types of stats, though, are called advanced for a reason--they're not for the casual fan to memorize and calculate on the spot, but they do have a lot of value for people who love to dig into the numbers and value precision over ease.