They are corrupt and don't address the problem because much of the union leadership are reactionaries opposed to people having real power. Unions used to literally fight the police and national guard, radical unions have been dismantled and co-opted continually for the last century. It's not just about holding power, it's about the underlying worldview of the groups in power as well, and any unions that meaningfully challenged the normal capitalist status quo have been crushed by the US security apparatus.