They do so because there is an economic incentive for them.
Decentralized will not do this, as long as they remain non-profit and strive to stay ethical.
This means that the average user will prefer the unethical and addictive platform. Same thing as with fast-food. Not only do decentralized systems need to be technically competitive and surpass hurdles that centralized systems do not have, but they also need users to be made aware of those issues.
They are necessary but utopic.