That's the whole purpose of a stock photo website.

To easily track down the owner of the image and buy a license to use it for money.

You can do it manually by contacting a person directly, negotiating a fee for a license to use that image, but that takes time. Stock sites speed up the process.

Just because the photo was not sourced from a stock photo website does not remove the need for copyright or payment.

It's like saying "I copied a dvd and didn't buy it from a dvd shop, so I don't need to pay for it".