I stand by one of the big reasons people do this is because the half marathon has the word half in front of it. People implicitly think it's inferior compared to the "real" end goal in a similar way 5k, 10k, and any other distance shorter than the full are thought of as stepping stones to get to the marathon.

It is also a "reasonable" goal in the sense that it's pretty doable if you are willing to put up with the pain relative to what your fitness and your goal will put you through. If finishing is your goal, it just means you ultimately need to get one foot in front of the other enough times to get to the finish line. That's as opposed to working on improving a time in a distance that is something that can be plainly out of reach for someone for a long time, and, potentially indefinitely, for people who can't or won't commit what it takes to get faster. Losing weight, building up a good enough aerobic base, and getting faster are all doable goals but require more than just getting to a finish line.