It requires some effort. You have to actually sign up and attend trades school (which requires a non-zero amount of money up front and you’ll go 6-12 weeks without pay - which also can be a problem for some people), then spend some effort finding someone to take you on as an apprentice, and you have to actually think and learn things. Nothing particularly difficult but still, you need some non-zero degree of critical thinking skills.

If you don’t have that level of motivation (which again, is really not that high), you won’t succeed. You can walk into an Amazon warehouse (or honestly, any similar warehouse job) and start working either that day or in very short order, and get paid in 2 weeks or less. It’s the least amount of effort possible to get a job, but it’s also the least rewarding (mentally, financially, etc).

There are also some people who genuinely cannot work in the trades. Almost any kind of physical disability and you’re at a severe disadvantage. Same with mental disabilities (to a lesser degree).

There’s real downsides too. Some trades are fairly dangerous (electrician comes to mind, especially if you’re working with oldheads who are happy to work on live unfused lines/etc) and almost all of them will result in you degrading your body faster than average (although this is also true for warehouse jobs). Buddy of mine caught a piece of shrapnel with his eye (while wearing listed wraparound safety glasses, mind you) while grinding some metal underneath a car, and has permanent vision damage in one eye at age 24. You also have to be pretty tolerant of more… abusive working environments. Aside from general tomfoolery (“boys will be boys” type mindset), there’s a substantially higher chance than average that your colleagues will be racist, sexist, homophobic, transphobic, or all of the above, which isn’t great if you fall into one of those categories. It’s a lot better now than it was even 10-15 years ago, but it’s still very much a problem. (I’ve never had an office worker casually drop a hard-r while at work, but I’ve encountered enough tradespeople doing so that I’ve lost count of how many. And not even in an even remotely acceptable way, just whites dudes straight up using it to refer to someone as a slave or similar uses. In the past ~3 years.)

And honestly, probably a lot of people working at warehouses are moving to trades. Warehousing in general is high-turnover (40-50%, if not higher), so it would not shock me if a non-negligible portion of workers are switching to a non-warehouse career. I don’t know how to find stats on that though.

In short, lots of reasons.