I just visited the Rolex website to learn a bit about what true, "guaranteed waterproof" requires - there's a nice, long presentation on the evolution of their techniques over the decades. There's just no practical way to do something like that for something like an iPad at something approaching a "popular" price. It starts by having a case-within-a-case construction, each of them sealed. And its iFixit Repairability Score? Forget about it! The rough equivalent is what people currently do with iPhones - an accessory outer case like Otterbox, with a water-resistant iPhone inside.
The fundamental question for me is, why would every iPad or iPhone owner want to pay the extra cost of true waterproof construction? Not only would it be more costly, it would also inevitably add bulk and weight. I'm sure those are "prices" that many users would rather not pay, because it just doesn't match their lifestyles or intended use.
Speaking for myself, my iPad doesn't go into the bathroom. I go there to take care of my personal hygiene and get out again - I'm not going to watch a video while brushing my teeth, and certainly not while showering (my eyes are mostly closed while showering, and it's noisy - I'd also need waterproof earphones as well, and they'd probably be knocked from my ears while shampooing).
The only other place my iPad is at risk of wetness is in the kitchen for displaying recipes. When I do that, it takes just a bit of care to keep it high and dry. An accident could happen, of course, but the possibility is sufficiently remote that I wouldn't pay for an Otterbox, no less a Rolex-equivalent iPad.
But certainly, other people are different. The only question in my mind is whether a truly waterproof iPad or iPhone (a specific model such as iPone/iPad Pro, rather than making every model waterproof) would be sufficiently appealing to enough customers to be offered by Apple, which, with very few exceptions (Mac Pro) sells mass-market products.
