The web shop timeout mystery

Another one of the things in the modern world I’ve not yet understood:

why on earth do some web-based shops timeout your shopping and automatically clear you “shopping cart” if you just leave it around for a few hours/days? Why why why? What harm does it do them if I don’t hurry on to purchase?

I love being able to press ‘buy’ on lots of stuff (that then are added to the “cart”) and then ponder a few days if I want more stuff, if I selected the right models, alter a few things and similar. So when they time-out on me like this, it’s like a blow in the face and I need to start over again. It’s simply crazy that I have to backup my list of things to buy just in case they’ll flush me before I’m done!

Yes, I’m aware that some sites offer “save lists” etc if you’re registered and logged in and all, but I don’t want to have to do that.

I can imagine that at times things run out of stock or they even change the prices of merchandise that’s in my cart, but they could still solve that in other ways than just clearing everything.