I guess they do it to appeal to their biggest customers - people who can't resist buying way more shoes than they need. But it does seem weird for some brands that aren't exactly fashion houses, e.g. Ecco, to do it.
It must cost them a fair bit of money too. I wonder how much cheaper you could make shoes if you only designed one set and then fired all of your designers...
My solution is to just buy a few pairs when I find shoes I really like. Reduces the problem at least.
Also I've started buying Levi's jeans - really expensive but at least they have some styles that don't change.