Re: space plane
Probably because they are actual scientists and are also taking into account mission mass and total cost issues as well as point-of-failure problems.
Adding a space plane will add mass that will increase the payload to chuck out of Earth orbit thus increasing cost. Additionally, it will increase the amount of mass to move to Titan, thus increasing the amount of fuel needed to push it, thus increasing payload even more, which increases cost. Finally, adding another element with another set of possible failure options that are extremely difficult to mitigate from a billion miles away and can totally render the entire mission useless is a risk that most true scientists, fully aware of how little money they are being allotted to work their magic, will most likely prefer to avoid.
Biting the hand that feeds IT
