Skin is airtight. It just needs an inwards force to substitute for external atmospheric pressure. Elasticity provides that.
The suit can even be somewhat porous to allow cooling by sweat.
It's a great idea. The major problem is no one has figured out how to build a suit which the wearer can squeeze their way into.
Also, The suit cannot have any concavities or "hollows", so you still need a certain amount of internal padding to fill them out. That's why cosplayers and actresses can never look like their comic book prototypes. The only way to achieve that "painted on" look is actual bodypaint.