If it was an aerial ladder, and the ladder was up, its not so simple to just drive away. The same with an engine that had numerous lines stretched from its pump panel.
Not having been there, I won't second guess, but most often you TRY to anticipate potential fire spread, both the where and how much.
I don't know anyone that would deliberately leave an apparatus in harm's way, since you can never trust that the piece would be replaced. Budget limitations are a reality, and one can never assume anything.