As a preface to this answer, please note that I can only infer (as I'm speaking on behalf of a lot of designer/programmer that I've never met)
conjecture: programmers are lazy
explanation: in code, it is easier to isolate sections of an interface. it keeps code clean, and files short, with limit dependencies. in order to have a zooming interface, the zoomed in interface could need to be tightly coupled with the zoomed out interface. this COULD make for some nasty code. (obviously you can code anything, so note the word COULD in all caps). iOS follows this paradigm often with UIViewControllers https://developer.apple.com/reference/uikit/uiviewcontroller/1621380-presentviewcontroller?language=objc In short, it is much easier to barf something onto the screen then it is to code nice animations and transitions
conjecture: zooming interfaces done poorly CAN lose context (and confuse the user)
explanation: if you zoom into too far, or too fast, the user may have a hard time of keeping track of 'where' they are and will feel cognitively heavy
conjecture: zooming interfaces don't fit as many paradigms as buttons do
explanation: when designing an interface, you are thinking about what makes sense. if it makes sense to emphasize going into and out of sections then go for it. Logically, this would make the most sense when you are traversing a hierarchy (animals -> dogs -> types of dogs), or spacial map (earth -> north america), or something that the user naturally associates with depth (like walking or looking closely). You can see how it wouldn't be adding much to Domino pizza's website.. order online -> (zoom) -> chicken, pasta, or pizza -> (zoom) -> pizza etc
conjecture: a zooming interface is slower than just changing the interface
explanation: if you've ever used a product made by an inexperienced designer, you'll remember the frustration the 50th time you press a button and you have to wait for their frustratingly slow animation to finish before changing the screen. animations in interfaces should be (for the most part) perceived, not noticed. <- if you disagree with this statement, feel free to comment with your favorite Power Point transitions (mines the checkerboard wipe :)