Except, it's not always an affront to their self interest. Unions increase wages for their members, not people who aren't in unions. This then tends to increase the cost of the goods they provide.
Unions tend to reduce overall employment, favoring the already employed over the unemployed. They reduce job flexibility, as benefits are often tied to long term attachment to a single company. Unions make the companies more vulnerable to outsourcing. Finally, the public employee unions in the US have conspired with politicians to enact defined benefit pensions that are bankrupting many localities- a problem of short term thinking. Unions essentially create a government sponsored monopoly on labor supply, restricting both workers and companies, which is fundamentally in conflict with the values of economic liberty upon which the country was founded.