Because it's a powerful tool. You are your DNA, literally. And when a company has so many information about people, some other actor will want that information

Because it's a powerful tool. You are your DNA, literally. And when a company has so many information about people, some other actor will want that information and becomes pricy. Like instagram. Why did facebook paid 1 billion USD for a company that generated 0 revenue? For the amount of users and information they have. And to twist a bit your question. Why is it so valuable AND sensible? Because imagine in 10 years, you get a kid and he wants to get a job, and because you gave your DNA away to some random company, now owned by the biggest employer of your state, they do not hire him. The reason? He has 5% chance of developing a sickness which might drain the medical coverage of said employer. We would be reduced to numbers and probabilities. Labels attached to our biology.