The difference with o-rings & gaskets and the like is they are not at wearable joints. They are put in place, and they stay in place, and there is no wear on them. The most worn seals in a DSLR are the seals around the buttons, underneath which is a full seal that separates the button itself from the electronics inside.
My point is that an articulating screen is something that will constantly be pulled out and put back, on a continual basis, over the life of the camera. The joint there is going to take a lot of wear (especially if it's one of those swivel joints.)
I am aware that it's been done. I would also be willing to bet that Olympus has encased the ribbon itself in the seal, rather than breaking the seal to allow the ribbon through.
I'm not familiar with the ways that Canon or Nikon weather seal their cameras. However, O-rings (but not gaskets) have and continue to be used to seal joints that move. You can find them on rotating and reciprocating machinery such as pistons, shafts, etc. The O-ring design itself isn't innaproporiate for such joints per se, it's that you have to pick appropriate surface roughness and O-ring hardness for the taks at hand and for the surfaces that form the mate. It's certainly mechanically possible to design a joint with a ribbon cable that is weather sealed, to whatever specification Canon builds it's camera bodies to.
Whether such a solution is appropriate and/or cost effective for a DSLR designed for a specific price point and performance level is another matter. I think we are generally in agreement that a moving screen probably won't be included in a weather sealed, sports-focused body, for obvious reasons.