Because many types of work do not lend themselves to be broken up into pieces. Communication/coordination overhead can be quite large as any programmer will be able to attest to. Also, educating twice as many people costs at least twice as much.

But of course that's a very narrow view on the costs and benefits of having a lot of skilled and motivated people around. Available work is not a fixed size pie after all.

I totally agree with you that the disappearance of a huge number of menial jobs is potentially disruptive. It's actually quite amazing how many people still do jobs that anyone can easily pick up within a week. These people are just not used to learning no matter how much politicians beat the education drum. That doesn't mean they're dumb, lazy or uncreative but that by itself will not get them a new job I'm afraid.