Because that's not the way it was designed and coded...the gradient is (I think) a simple vector drawn gradient over which the the gradation occurs. The brush is a painted mask that must be stored parametrically. It doesn't actually create a mask, it stores the mask in X Y coordinates and then uses those numbers to render the mask during processing. Both functions, the gradient and the brush are totally different concepts in terms of describing the way an adjustment will be applied locally.
The way Lightroom is presented makes us (users) prone to not care about how it is coded. That is a great quality! It also makes you wonder "how could things be even better for the user", instead of "ah, I understand why the application has this quirk because the underlying algorithm has to ..."
Eventually, no matter if it is a vector or a bitmap, I assume that the parameters are somehow transferred to a "fixed" image processing pipeline. If the gradient and the brush are both affecting WB, then I assume that they are somehow merged into something that can control the WB-processing-thingy, instead of doing two pixel-affecting WB-processing-thingys in series.
The reason I am requesting this is that I often find the application of brush tedious, time-consuming, and difficult to align to large simple geometric shapes. The gradient, on the other hand, is very coarse and always straight. I might like to make a general outline of the horizon using something like the gradient (or perhaps a smooth low-order spline), then use something like the brush to add/detract to it to work my way around trees or other irregularities. Then I'd like the two of them linked to e.g. pull down the exposure of my sky. Perhaps there is some neat work-around inside Lightroom that I did not think of?
...there's this little application called Photoshop...you may have heard of it?
Tried it... not anyhing for me. I can code basic image processing easier (and a lot more satisfying) in MATLAB than I can navigate the frustrating endless popup windows, minimal icons and OS-alien interface concepts. If I can't do the simple stuff within an hour ("hello world"), I seem unable to motivate myself to work until I am able to do the fancy stuff (and I am impressed by the stuff photoshop-jockeys produce). Photoshop feels to me much like Wordperfect did in the late 80s: a mature product that did everything the core users expected it to do and blistering fast, but soon to be beaten commercially by the (in many ways inferior) Microsoft Word that made basic chores simpler for basic users.
Perhaps I am just lazy and a cheapskate...