This rings extremely true based on my observation. Fundamentally, companies working on large projects will ship their org chart. It is modestly more difficult, if your org chart gets spilt in two such that two or more teams are now maintaining the same SPA, to synchronize their work such that they don't interfere with each other's activities. You begin looking at a more traditional application development model with full program integration testing in the mix.

Multiple pages means multiple entirely separate client-side contexts that are a security hole if they can clobber each other.