There is overhead on jobs. Unless performing the most simple mind numbing factory assembly line work, 2 x 20hrs << 1 x 40 hrs. There's also double the time wasted on commutes.

Then there coordination overhead in the family - even if the two parents work 20 hours each, it's unlikely that it's exactly divided so one is always home. Child care costs a lot of money. Eating out costs money.

But the most important reason: People still want careers and a career on half the working time takes in extremely simple terms at least twice as long.