Unions are for situations where individual workers can not negotiate well. That's why the term "collective bargaining" is often interchangeable.

IT workers are typically considered professionals why can negotiate for themselves.

The general rule of thumb is how "replaceable" a worker is. Someone like a doctor or a hard to find engineer is considered professional, and thus a union is not needed.

Someone who has a lot of handholding on their job, and can easily be replaced by someone else who will train quickly, is what a union is for.

Thus, a union might work for the cleaning crew, foodservice crew, and maybe even Waymo testers. It won't work for software engineers, because engineers should be able to negotiate for themselves.