For the same reasons why we eventully pollute and corrupt every system and environment we use. If there is any benefit that can be extracted for some while the costs are borne by many, than this will occur and generate a positive feedback loop that grows over time.

It's the law of monetization.

For money! Ads make money.

Because while consumers value “inefficiency” (high design, wonderful prose, beautiful images, great usability) they don’t want to actually pay for it. Producers have to become extremely efficient without revenue, and are stuck with a choice: Produce at a loss, stop producing, or seek payment from another source (sponsorships, ads).

It seems there's little agreement over how the web is broken.

In order to break the user, of course.

To improve the user experience.

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