Bungie said that there was originally a level planned after "New Alexandria" where you have to escape the city post-glassing. One idea was to board a Scarab as NOBLE team and use it to escape the city.
For some reason, they didn't think the gameplay would be all so good....
Two reasons for that:
1) These scarabs aren't vehicles. They're a massive version of the Hunter, controlled by Lekgolo worms. We can't drive scarabs for the same reason we can't take a Hunter's fuel rod cannon.
2) This is the more obvious. A Scarab controlled by a player would blow through almost anything. Although, now that I think about it, only having control of the one Scarab gun would make accuracy a necessity, and leave you open to attack. They'd have to make the scarab able to be destroyed without boarding, though.
Well, think about this. The Lekgolo are living. If you could study what pulse or flex does what, you could touch and push the worms into a certain path (pretty much torturing it into moving a certain way) and control the Scarab. Well, maybe not
control the scarab, more like blatantly moving it in a certain direction. Or could you?
BTW:
2.5) Bungie got lazy and didn't want to put in sexy scarab textures.
