Question

Why does a spacecraft have to be airtight?

Answer 1

it should be airtight because space is like a vacuum, and a
non-airtight spacecraft would vent the atmosphere, making it impossible
to breathe

Answer 2

In space, air particles will try to distribute, as there is nothing. This will result in the gasses flying out to space. The ship has to be airtight so that the gas will not escape.