If one believes the conflicting information that's been bounced around over the past decade few years, finishing SQ42 is supposed to bring players into SC proper. Additionally, the PU is dependent on a lot of the tech being developed for SQ42 (or vice versa, or they're codependent, like any healthy relationship).
And selling a star-studded, fully-realized, single-player game that deposits players into the land of dreams and what could be, is a great marketing technique to open wallets.
"Look, we showed you how good we are with the ride you were just on, now imagine how awesome all of this (the PU) will be? Please enjoy the gift shop, and pick yourself up a Legatus Package on the way out. Remember, this is your chance to get in on the ground floor - just like everyone else for the past ten years."