Maybe because that's the way life evolved in that case

As for beetles, well, spaceships tend to have squishy humans inside too

Spidersilk is merely an example, it's designed to do a specific job, which is not to hold armour plating together, however, it's only a question of how it is laminated and hardened that determines the directional strength and shear capabilities. Spidersilk would be no good, but something
like Spidersilk would be.
It's easy to think that Aliens would think like Humans and assume that building ships is far far easier than growing them, but that's only because we
only know how to build ships, not how to grow them. Other life forms may have far more knowledge of Biology than us, possibly at the expense of manufacturing knowledge, maybe for them it
is easier to grow them, and they can't figure how anything constructed could survive in space.
All I'll say is that, just because we don't know how it's done does not always mean that it is not do-able.