Because perfectly executing our imagination is hard without a massive budget. And because:
The trick is to do the work before the expensive production machine is awakened.
also have to remain pragmatic and flexible, work with what you are getting not what you wanted, or you lost just as well.
But there are countless artists that will never be happy with any of their work, ever.
Many recognize this and utilize it in a pragmatic way, to a great benefit for all.