Time is not all you need.

To make something, you need skill, time and materials.
And the "something" is really what you are after.

In a very simple world, you would trade your something for something-else.
And that trade would be even.

Just trading time, or some voucher for time does not get you the "something"
There is more to it than that.

And time vouchers can be horribly abused.
Like, you could own someone's time for the rest of their lives.
Or slavery.