I work in the corporate world. The reason it's frowned upon is because employers rather see complete projects that you did rather than a bit of random code here and there in an open source project.
"the corporate world" is a pretty large place spanning companies that will hire people based solely upon open source contributions as well as companies that will disqualify people based solely upon open source contributions.
It's a pretty meaningless statement to say "in the real world, no one cares about open source" when your definition of "real world" is one fraction of one fraction of the real world (specific companies within the corporate world).
It's a pretty meaningless statement to say "in the real world, no one cares about open source" when your definition of "real world" is one fraction of one fraction of the real world (specific companies within the corporate world).