The main reason is advertising, which is ultimately how this web content is paid for.
But there could be alternatives. We could create static img elements that could securely and discretely record whether they were viewed by a human (to prevent ad fraud). We could support simple animations and interactivity in a secure, resource-friendly way. And we could allow these to be disabled by user agents (e.g. epileptics).
Could newspapers exist without online advertising? Possibly, but only in specialised niches. Consumers have shown that they will always seek the cheapest, most accessible forms of information - even when provided by highly biased, unreliable sources.
The danger to killing online advertising is that it creates a world where the only people who can afford to publish are those with other interests - plutocrats, politicians, churches - and who will inevitably bankroll news only to further political ends.