Unions aren't structured for investing, and that's not their reason to exist.
I don't know where you are from so I can't comment on how unions function in your society, where I live they're definitely not in the same realm as a potential investor for cooperatives.
You are also asking for unions to compete against VCs, and banks, let's be realistic that even a large and wealthy union is not even close in terms of wealth management as a small to mid-size bank/investor. On top of that the union has responsibilities to provide benefits to all members, a bank or investor has none of that, tilting the scale even more in disfavor of an union assuming the role of investment.
It's a good thought though, another entity such as a credit union supported by workers in an industry which also takes the role on investing in cooperatives to foment them, would still be an uphill battle but if some good cooperatives came out of it maybe it could be another viable (albeit smaller) model to start companies.