The short answer is - because your feedback is so valuable to the company that orders the test. Your inputs are combined with all other testers’ inputs, and the result is delivered as a complete report, which the client pays for. Some of that money goes to you and the other Testers.
In some cases, the testers we need are difficult to find. Therefore, we raise the compensation amount This means that what you get for participating in a test can vary significantly. If the compensation for a test is unusually high, it is likely because we are looking for a very specific or narrow group of people.