Private corporations also have a pretty consistent thread of corruption, a similar example to the one you gave being HSBC laundering money for drug cartels. Again, pretty comparable.
This isn't to say corruption in unions hasn't been part of why they've declined, its almost definitely part of it. While a corrupt corporate leader generally creates more money for its stockholders a corrupt union rep often costs the rank-and-file members which can greatly sour their opinions of unions as a whole. But there's nothing inherent to unions with regard to corruption, people in power often take advantage of their position no matter what type of structure they have that power in, and the point that it appears, at least to some people, that unions are held to a different standard than other types of organizations is worthy of being discussed.
However, after that corruption was cleared up and the pensions moved to Goldman and Northern Trust, the funds were destroyed and benefits to retireeswere deeply cut.
https://www.marketwatch.com/story/how-the-teamsters-pension-...
I had to keep pausing and checking if something was real as I couldn't believe how bad things got
The most famous example is Jimmy Hoffa, whose story has been made into films... including one in 1992 with Jack Nicholson.
“ Hoffa became involved with organized crime from the early years of his Teamsters work, a connection that continued until his disappearance in 1975. He was convicted of jury tampering, attempted bribery, conspiracy, and mail and wire fraud in 1964 in two separate trials.”
One example of many.
> This sort of thing happens in private corporations
Private corporations have a mission to make money. Unions have a completely different mission.