... Why can't Olympus make a weatherproof E-620 with a large viewfinder?
The optical viewfinder system on a single lens reflex is a very expensive part of the camera and has its impact on the cost and design of the entire camera.
If you want a large, bright optical single lens reflex viewfinder with a 13x17.3 sized format, it requires a very large (relative to the format size), expensive prism and top-notch optics to do it. Which is out of the price bracket of an E-620 class camera.
The E-30 is a watershed in this regard. It has about the same size and magnification viewfinder as the E-1, a little brighter, and costs substantially less than the E-1 did when it was new ($1100 vs $1800). Note that the camera is about the same size as the E-1. You can't fit that optical system into a camera the size of the E-620 without raising its price and enlarging the camera substantially.
The E-3/E-5 reflex optical system is substantially larger, more efficient, and much more expensive to manufacture than the E-1/E-30's reflex optical system. It's the reduction in cost of the other electronics components and the lack of a weathersealed, magnesium body/chassis that allows the price of the E-30 to be so reasonable.
(I've just about made up my mind to order the E-5 instead of the E-3 I was going to buy now, even though it will cost a hefty premium. Just have to wait for RRS or Kirk to have the tripod plates and Adobe to update Lightroom and Camera Raw before I buy.)