Thereby creating an incentive to accept as many people as possible, thereby lowering standards, thereby hijacking the agenda towards protecting incompetent workers.
If you're unionizing a low-skill segment of the labor market, I don't see a feasible alternative. Even if you only accept the best and brightest, the best and brightest are still trivially replaceable at that level.
There are still classic craft unions, with apprenticeship and training programs, but not many are left. The International Brotherhood of Electrical Workers is still around. The Plumbers and Steamfitters have apprenticeship programs. The Animation Guild (Local 839, IATSE) represents animators at the major Hollywood studios. They give classes for members, both on using new technology and on classic techniques. Read The Animation Guild's contract, which applies to most animators in Hollywood.[1] This is for a job more creative than most programming.
[1] https://animationguild.org/wp-content/uploads/2016/05/2015-2...
Pro sports players' unions.
It makes sense -- owners must act as a collective unit in order to ensure competitive balance and some degree of economic stability. A sports league where one team can afford to buy all of the best players from other teams is going to lose fans very fast. With owners acting collectively, players act collectively as a counterbalance. They make sure that they get a reasonable share of the overall income, and that it's distributed in a way that pays top performers more but also pays well enough at the bottom to keep the league an attractive place for marginal players.
On the other hand, the New York Yankees.
Those that would benefit most in theory from unions generally have the most experience "in the trenches" with them. This experience shows them that unions more or less became places to protect the lowest common denominator with zero regard for any value they bring to the table.
Show me a union that functions like a "guild" where it polices it's membership extremely well, and people strive to be skillful enough to join - then I will both support that union as a member, and hire those members when possible.
If all you exist to do is keep the lowest performers employed at the expense of your most productive members you can see why most hard working folks tend to have a complicated relationship at best with unions as they are implemented today.