Because it would be cheaper to use a machine to build the bridge, give the money you save to the people, and then let them add value to society in some other way.
But the economy doesn't want them, or is unable to utilize them under present conditions. Eating what you have in front of you is still more filling than waiting (or letting someone wait) for that next, bigger meal.
Of course, that also doesn't mean we shouldn't eliminate the incentives that contribute to inefficiencies being preferable. There are plenty of scenarios where people are employed to dig a proverbial hole and others are paid to fill it (patents and copyright enforcing intellectual poverty; interest on public debt, etc), even if each side spares no expense and might use the best tools available. If we were actually solving problems, prices would be going down and people would be happily unemployed (because there were less problems to resolve and less to need money to buy) at the same time as quality of life were going up. That is not what is happening.
He said "let them add value to society", not the economy. The two are not synonymous. If the economy is unable to make use of them under present conditions, then giving them a job is useless. Giving them a basic income lets them make non-economic contributions to society.
You're assuming the value of a person's time to themselves is zero. If it's positive, and if the economic value we're deriving from the make-work is less than it, then the make-work program is actually destroying value.
But the economy doesn't want them, or is unable to utilize them under present conditions.
Capitalist corporations don't want/are unable to utilize them. Don't confuse them with the economy which is far more grandiose and inclusive than that. There are millions of ways for humans to create value for other people, most of which would be unprofitable for capitalist corporations to pursue.
Sure. That other thing is self improvement, looking for normal employment and helping their family / community or whatever else they think is important. In other words it is a basic income for the unemployed...
