Plus the fact that then you need the everything to be structurally strong to be able to support 1atm of pressure differential, keep the vacuum sealed, and keep material degassing under control, so you end up with some heavy duty, expensive stuff. Pretty sure a lot of steps in the chip fab are done under very very high vaccums tho!
In general it's easier to control the air you're blowing into a room than what you're extracting, so I'd expect it would be more economical to keep the fabrication rooms under high pressure than low pressure. Also, in case of a leak, air and contaminants will flow out of the room instead of into it.
