Care to name some of those similar platforms?
Do those platforms also offer a "direct to you" option, which means that the seller is the merchant. It would be interesting to see how they solve this. Do those platforms have a wallet system? Do you need an account to buy there? You do not need an account on Itch to buy things.
Sure, all this could be solved, but that would take development time, since they cannot use a shop system out of the box. If buying different things at the same time would be more common, they might have more incentive about having a shopping cart. But as it is, not even buying from the same seller at the same time is that common, so that the plans for bulk selling were implemented. I guess developers just use the bundle system to group items together.
There are many shops for established games and if you search such games, you will find alternatives. But apart from that, I have never seen anything like Itch. Gamejolt seems to have been similar, but they changed to only accept creators with over 1000 social media followers among other things.