A watch mechanism is (typically) circular, ideal for devices that need to be opened and closed frequently. And the mechanism is a much smaller percentage of a watch's overall volume; much more of its volume is enclosure material.
I can't find the rating for the 1926 Rolex Oyster (if there was one), but the Submariner in 1953 was 100m.
... I daresay we're not talking about that level of waterproofing, so circularity of sealing gaskets and case structural integrity shouldn't be as critical.
Although if Apple made an iPhone Deepsea, I have no doubt they'd have buyers at any price point, a vanishingly small percentage of whom would actually submerge it.
Circularity allows for threads which can give significant, robust and uniform clamping pressure, without need for any adhesives.
But really the distinguishing factor is the ratio between case volume and mechanism volume. If phones had a similar ratio to watches, you could just bolt them shut with a dozen screws.
Ah, I get why you were emphasizing circularity now. Good point.
I guess phones do have the benefit of non-contiguous mechanism requirements, in that you could through bolt the lid into the case if you designed boards and components around the posts.
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