Except even with a contract there's a great deal of grey area that can allow those hiring contractors to futz with (I'm going with literal general contractors for construction here, but it applies to other fields). Unless you have a legal team on your side, the fine print is going to be lost on you, and guess what... that's something unions provide.
I was part of UFCW years ago, and had several negative experiences with the shop boss and management, mostly that they were protecting someone that drank on the job then ran a forklift, broke freight palettes all the time (forcing us to repack them, something that is NEVER fun), and were generally behind on freight schedule. The union protected this dingus, and this really soured me on unions for years. Enter the teacher's strike and NEA; teachers were getting shit on by everyone, other teachers, admin, and especially parents. Yes, teachers should be answerable and beholden to parents, but when you hear comments like "yeah well, you're just a babysitter with a degree" (not to myself, but directed at a friend at a PTA meeting), it's disgustingly patronizing and ignorant.
NEA helped organize the teachers, protected them from backlash from admin (usually failure to renew contracts for bs reasons), protected provisional teachers, ensured benefits remained untouched, and forced through a COL raise that most districts had been refusing to grant. Yes it still is far from perfect (Arizona, I'm looking at you), but without unions you'd massive turnover from even the most dedicated teachers, creating a revolving door that benefits no one, the students the least of all.