In my current experience, clients are mostly non-technical and have no idea of the importance of documentation, what is an automated test, or what a CI/CD pipeline is for.
You can perfectly develop software without these things. Just test things manually, deploy from your computer, and memorize a few commands to develop locally. I'm also not saying fixed-fee will result on complete garbage software, it's just that there's no incentive to go beyond mediocre.
It is going to make things worse for the next freelancer or whoever maintains the code, not you.