You are thinking too small. Often, the problem is too difficult to do the work if only one customer would pay you. You need either many customers, or to charge more than one customer would be willing to pay. There is the option of starting with a minimal subset and adding features over time, but this can be very difficult, especially if the minimal subset is still quite large.
Take large accounting software as an example. There have been many one-off solutions made for the big companies that could afford them. However, nowadays many solutions are customizations of a common platform. The cost of implementation is lowered because the platform maker can spread the cost of common features and updates across multiple customers, and the customers are happy because the quality and flexibility is better. The common platform can gain large features anticipating future needs, even if no one company would be willing to pay for it.
The economies of scale for software (and games, and books, and...) can only work if creators can sell their product more than once, and copyright is the best tool we have for this.
Now, if copyright terms were reduced somewhat, it could still be possible to get your initial investment recouped before it was a free-for-all, but changing the law would need to be done carefully.