It's been a few years but my recollection is something like:
1. The jobs that are crappy enough for jobs programs are not useful jobs to be done anyway.
2. The overhead involved in having a job (Transportation, childcare, all kinds of second-order negatives on your life) can easily outweigh shitty salaries. Also jobs programs would be worse for people with kids, unless you add more child tax credit to prop up that side of the stool.
3. Having a job takes away free time that otherwise could be used for training or education. Planting trees by hand in the sun is not going to look like anything on anyone's resume. You could put another leg on the stool by having a college grant program or something, but it's another step away from jobs and towards UBI
If you're going to pay people to do something pointless, maybe just pay them to exist anyway?