Every SoC has a boundary. Signals have to get off the die at defined levels, with defined drive strength and protection, and memory interfaces have to meet timing against parts whose behaviour is specified by someone else.
This layer is easy to under-plan precisely because it is standard. The failure mode is discovering late that the required I/O type or memory interface is not available on the chosen node, or is available with characteristics that do not suit the design — at which point the options are a node change, a redesign or a compromise.
Which is why I/O and memory interface availability is one of the inputs to node selection rather than a consequence of it.