A spider spinning web is an emergent behavior built up from a bunch of simpler ones like excretion, simple movement, and electrostatic hopping from point to poi

A spider spinning web is an emergent behavior built up from a bunch of simpler ones like excretion, simple movement, and electrostatic hopping from point to point. A bird flying requires the coordination of a much larger number of muscles at the same time, which is much more complex from a nervous system perspective. Spiders reproduce much faster and have a much smaller survival rate so they're well selected for that kind of instinct. Birds less so.