Because 99.9% of typical workers don't have the money to build the business.

Take for example, Google Inc. origins in 1998. Both L Page and S Brin came from upper middle class families but neither of their bank accounts nor their parents' bank accounts had $25 million to spend on a business. It's also doubtful if either family could have been approved for a $25m unsecured business loan from a bank. Likewise, Google's first 100 employeees also didn't collectively have $25 million. On the other hand, VCs like Sequioa and KPCB did have $25 million to invest. Later on, more investors bought IPO shares totaling $1.9 billion. There is no union of workers that can pool together that kind of cash.

Since a co-op by definition must be owned by the workers, the question is no longer "interesting" with a mysterious answer. Instead, it's tautological. If you look at the typical workers' modest savings as the ceiling for funding a business, it's obvious why they can't compete with businesses built by investors/capitalists money. While WorkersOwnedSearchEngine Inc scrapes together $30,000 from their owners-workers' savings accounts to buy a 5th rack server from dell.com, Google Inc. can use a fraction of its $2 billion war chest to buy an entire data center and also break ground on a brand new one.