Why are most commuting panniers designed as shoulder bags and not backpacks?

They exist, but they're hard to find in real life, probably because the market is small. All the ones I've looked into would take a large laptop, but I've only tested with little netbooks. There's at least one fairly recent review online.

I'm in a similar position: sometimes I need to walk or even run with my pannier (also I switch between bikes daily). I need to carry work clothes but not smart ones. I've had two backpack panniers both in the 20--30 litre class. That's enough for a change of clothes and a laptop, plus cycling essentials, and even a gym kit, but a (large) pair of shoes is a squeeze.

The cheap one (£20--40 £/$/€ on eBay/amazon) was quite big. It had some nice features, but converting between modes was slow and relied on removing a plate that held the rack hooks. This wasn't as secure as I would have liked -- it fell off a few times until I reinforced it; the rack hooks also needed replacing with stronger ones (Carradice) after the flimsy plastic failed. A year of daily use and the fabric is going on the corners. The straps and back pad zipped away when not in use, which kept the mud off, though they still got a little damp and some dirt from corners could get on clothes.

The new one from Altura is made of much tougher fabric and is much easier to convert. The backpack straps are on the outside when riding, away from the wheel and therefore the worst of the dirty splashes. This also means no adaptor plate to lose. This came from a large (chain) bike shop, but they only had the one type in stock. This meant I could try it on (but not on the bike which was in the wrong city). You may need to decide whether you want a left or right model as some are tilted to avoid heel strike. I got this wrong but can get away with it on a big frame. This was good after a very early warranty replacement for a failed main zip, but the zips are clearly a weak point because the main one broke after a couple of years. I'd like to replace it but that looks very difficult so I'm back on a cheap backpack

To return to the headline question: many people settle on a backpack (or even a shoulder bag) for this type of need, reducing the size of the market for convertibles (which also need more design and hardware so are more expensive). I could in the winter but the extra sweating rules it out even in mild summers we have here. You can of course stuff a backpack in a front basket, or strap it on to a special handlebar rack -- the latter would probably be better for a laptop.

I'm sure many people (like me) return to their desk at the beginning and end of the day and can dump the pannier there. I keep a different bag (conference freebie) for when I need to carry more than a handful of stuff around the site. While this may not suit you, it further reduces the market. In fact my laptop shoulder bag pannier has an insert sleeve that did this job very nicely when I rode door-to-door rather than the convoluted commute I have now.

Something to look out for when choosing from the few options that do exist, is how well do they carry a helmet (assuming you don't want it clipped on with its own strap and swinging around). My old one can fit a helmet in the outer pocket only if the whole bag isn't too full; on my new one you can tuck the helmet in to the pocket formed by the cover for the rack attachment points however full the bag is.