Why not integrate an open-source system like Firefox Sync?
Because it's a total mess. I've worked on a protocol re-implementation for personal use, and I assure you it's quite ugly. And it's a moving target that changes once in a while, so supporting that is essentially being at Mozilla's mercy. Mozilla's Kinto could do the trick, though. I haven't dug inside but protocol-wise it looks quite good to my personal tastes. I believe this is what actually Mozilla wants to eventua…
It's not that bookmark-sync is so important for most people that they cannot wait some minutes till a periodical checker fetches the updates.