Regarding the question why scientists don't buy the sequencing machines themselves I think a major factor is likely that sequencing is something that everyone expects to be a service and not something you do yourself. I would not even have considered doing this in-house.
At $15 per plasmid there is also no real advantage for typical labs, they're unlikely to save much money here. It could be quicker because you don't have to send the samples, but that likely isn't a big factor here. And having another device that either someone has to run as a service locally or that you have to teach everyone is a rather big effort.