Because you pay a tiny fraction of the work spread out over a long time (longer than "today", typically) and once the trade secrets get out competition tends to go up, prices tend to go down, and the ability of future sales to finish paying off the development work (or paying for the next development work depending on your accounting model) gets broken.
Oh, do they? AFAIK no one besides two peoples to this date know the real ingredients to Cola..
When I buy Cola, I pay for the work of developing new sodas. Why can't we have all their trade secrets?
Because a physical product is different than a non physical "product".
When you pay for a non physical product you already have paid all development of product. For example the product costs 2 million and people have paid 3 million .Why society cannot use the product freely?
You don t want trade secrets of software company .You want to use the software freely because you already have paid for the software.
It's like to buy a cola and Coca Cola forbid you to use the product to clean things.
Society cannot pay licenses forever ( 90 years is too much time)
People paid a lot for windows 95,and Microsoft will never release the software for public domain.