Why do Unions ruin companies?

The Dock workers did it, the Miners did it, and now the Royal Mail workers are going the same way. The Vauxhall cars workers have opted for a cut in staff total and a 2 year wage freeze. At least the company will still be in operation, and in the future can build up the workforce again, a very sensible way of doing things. The Royal Mail could lose 65% of it's contracts and still the Union puts it's head in the sand, or is waiting for the Royal Mail to shed half it's workforce and then privatised. The mind boggles at the stupidity.