Can someone explain

I must admit that I do the same myself (when it's safe to do so). With arthritic knees and hips, plus spinal deformity which affects my balance, I have difficulty walking across the frequent slopes on the pavement, especially if I've not got a stick. Where the pavements are flat, that's not a problem.

Most likely because the pavements are covered in Dog Sh*t

I walk on the roads for the same reason as Jim

Regardless of why you do it walking against the flow of traffic is the correct and sensible way because it means you are facing oncoming traffic and therefore should at least have a chance to leap for your life, with your back to it you'll never see the car which mows you down !

May have something to do with the fact that most pavements have massive trees plated in the middle of them.

Also, even a small patch of water can make pavements very slippery. Particularly the tiled ones. Wherever you walk here, you need to look where you are going. It would be good if people looked both ways before crossing the road too. Some of them just walk straight on to the crossing oblivious to what vehicles are around.

Depends where you're talking about ... In many places, I wouldn't call them perfectly good pavements. A lot of the pavements are tiled and can be slippery on slopes. Other places are uneven and have chunks out of them where there's a lamp post or telephone pole situated and not forgetting the dog sh**!  To walk against the flow of traffic is the correct way - was taught that as a child. I don't think, given the choice, people would walk in the road for no good reason, but some of the above might be an explanation for this madness.