Imagine that you were a musician. How long will it take to make a song and observe the reaction of listeners? Can you figure out what is wrong when the reaction of listeners are not as expected?
How can we learn about product development that customers appreciate? To verify your understanding, you need to make an actual product and show it to the customers. The verification is expensive.
We have only one life. The observation of your own life is only once in a lifetime. When you die, you may know whether your life is a good life. However, you do not have the chance to use the experience.
Some domains are easy to verify, and others are difficult to verify. The domain that is easy to repeat experiments and easy to control environmental factors that affect results are easy to verify. Science and engineering have developed mainly in domains that are easy to verify by experiments.