Companies hire junior devs (and other cheap labour) as "reserves", in the off chance that you get more projects (sometimes this is negotiated as a bonded contract, which you can't break for 3-5 years, but I hear that abusive practice is dying slowly).

They are paid abysmally low salaries, but youre not allowed to work, or find work elsewhere while you're on this type of contract. If a project comes and you're needed, you're put on a regular contract that is comparatively not as low paying.

All the factors you mentioned are still at play, these people are almost never put on existing projects, so you end up with less people doing more work, with more people just sitting around doing nothing waiting for new projects.

This type of environment is extremely negative and depressing to be in, and it promotes a lot of office politics to get yourself off that list and into a better salary etc.