"Entertainers and sports professionals have unions which do not seem have this problem. "
Entertainment and Sports unions exist for completely different reasons.
Traditional unions exist because of an asymmetric power dynamic between workers and labour, at least, historically.
Sports Unions exist so that fairly highly paid athlete's can squeeze even more money out of their employers by gaining leverage.
Actors work in an aspirational industry that is massively oversubscribed with talent, and wherein the work is generally ad-hoc or contractual and conditions vary greatly.
The vast majority of actors earn very little and probably are not full-time actors. Their Union sets a 'floor' for basic comp and expectations because otherwise there wouldn't be one - but they generally don't get into controlling roles for talent, they don't control job definitions etc..
I have a neighbour who worked at an auto-plant and his job was to sit in a room and does nothing, observing the floor. A role with kafkaesque bureaucracy and roles that simply should not exist. Family members have priority for jobs. In a nutshell - that union has considerable control and influence over the operating nature of the company whereas I don't think that's a long term benefit of unions.
My sister in law works for a government sector union and their hiring practices have almost nothing to do with merit or qualifications, it's shocking. Again, a situation wherein I think the power of the union is a detriment to the system overall.
That said it seems clear that after a wave of 'getting along' in the US, surpluses are not being shared and unions at Wallmart, Google, Amazon etc. seem like a good way to develop better equity. But I suggest it would be better for everyone if those companies just did a better job with their staff and were more gracious and generous.