It's a valid comparison because Apple Watches are computers called a "watch". They include a monitor and smartphone. Yes, they are portable but so are laptops. How many round laptop screens are there? The Apple Watch is wearable but that doesn't mean is has to be circular. They have far more functions than just a watch. If they were only watches, round will work. If iPhones were only phones, they could be all sorts of shapes or sizes. They wouldn't even need a screen. Rectangular screens make sense as giving the most visible area for their dimensions.
Round watches are a legacy of analog clocks. In fact, no one really knows why clocks were designed and popularized as round. People put forth various reasons -- days are cyclical so we want circular timepieces, the round shape mirrors the shape of the sun, sundial shadows move in circles [best explanation] -- but we really don't know why clocks were popularized as round. Analog clocks do not have to be round but that's the convention. It's a design bias. Circles are pleasing objects to many people. They are interpreted as infinite (although they are not more infinite than a square -- you can trace the perimeter of a square forever if so inclined).
There's no reason for digital watches to be round. They can be but it's an inefficient use of space. Making them round is form over function. People complain about Apple deciding form over function but at the end of the day, Apple wants functional products. I like the look of round watches but it just isn't the best decision for smartwatches.
If Apple only released round Apple Watches, I'd buy one but that's because it has utility to me. However, the rectangular face is the more efficient design. If Apple made a round watch and a rectangle watch, I'd choose the rectangle one as more space-efficient. I don't use the clock faces that mimic analog watches. They look nice but I use the watch for its functions, not the look of it. If I want a "dumb" phone, I'll buy one. If I want a "dumb" watch, I can buy a round analog one or whatever form people make. Those are my biases but the mathematics of face shape shows that a rectangle gives more screen area than a circle does (21.5% more for a square versus a circle with side same length as diameter).