The Applied Phlebotinum of the film is a circuit capable to generate a force field which can be translated and rotated with a computer. On the other hand, the so-called "Thunderroad" was a hunk of junk that did not provide any function whatsoever. So why did the kids need to steal the Thunderroad before going to space? They could've just sat inside the forcefield with the computer and the battery, and they would've experienced a much more spectacular 360° view.
They're kids, making cool things without necessarily thinking about the practical aspects is what they do.
A spherical forcefield would not have provided them seats, or compartments to store their snacks, or a flat platform to mount the computer interface on. They really needed to build something to go inside the forcefield, so why not build their own space ship?
And of course, the Doylist answer: It would probably have been impractical, if not outright impossible, to pull that off with 1980s FX tecnology without looking incredibly cheesy.