Let's put it this way. With the Scarab's cost and long build time, making it to where three spartans can hijack a scarab would make the unit useless. UNSC players would constantly go after any scarabs with Spartans and the Covenant players would be too paranoid to build them, removing their most powerful, non-hero unit. While you're at it, why don't you make it to where 8 grunts can jack a vulture.

Seriously, making it to where multiple Spartans can jack uber units would make them too powerful. Why don't you think of both sides before proposing these ideas.