Two in one - now you're talking a big 24 pin+power pins+control pins chip. All the signals have to come to that area also.
And that only handles 2 buses.
If you have more than 2, seperate parts make way more sense.
If you're designing for 4 layer, 8 layer, 16 layer cards, there is way more flexibility in routing too. Parts can be spread out more for routing to more connectors.
A simplifed 1 layer routing doesn't really reflect much.