People who are not actively looking for work are not included in employment numbers (at least in the US). People voluntarily taking time away from working are not the reserve army of labor that our policies create to suppress worker power.
Attempts at full employment were tried, it resulted in dead economies. This stuff is basic!
Socialism doesn't require forcing programmers to be hairdressers. That's a huge strawman.
For the rest, sure, there are some who search but cannot find work. But what that usually means is they can't find work that matches their specialism. Maybe they could go stack shelves for a bit, but if your prior work was as a senior lawyer it makes sense to take the time to find a job that matches your skills.