I do not know the rationale, but this is the flip side of collective bargaining: if someone makes a bad deal on my behalf I can no longer deal directly with my employer.

Hard for me to imagine any cases where the union does a worse deal on your behalf than you did yourself. Maybe you're talking about only compensation? Unions bargain for more than just compensation: better healthcare, parental leave, more sick days, caps on hours worked, etc.

In aggregate unions exist to get a bigger share of a company's profits for its workers. (Instead of executives and investors)

But if you have examples, I'd love to read about them.

Bargained away? Which union did that and for what company?

When I started at Amazon full-time employees got a stock unit (once vested), when I came back, that was no longer an option.

That’s because of the lack of a union. Companies can’t just remove benefits that are in a union-bargained contract. (Assuming that would have been one.)