Because our bodies exert counter pressure and prevents it from bursting.

Because the pressure inside your body is the same and cancels it out.

The entire human body, including all the bones, tubes, sacks, and fluids in it,

is constructed to develop an outward pressure that compensates for the

inward atmospheric pressure.

Which is one of the main reasons why you don't want to take off your space suit during

a space walk ... your body could possibly inflate to the point where it would be tough

for your buddies to squeeze you back inside through the port. (In addition to other

space discomforts, of course.)