Because the authors of the original content deserve recompense for their work.

That's what the whole copyright and patent regimes are designed to achieve.

It's to encourage the creation of knowledge.

US Constitution, Article I, section 8:

    To promote the Progress of Science and useful Arts, by
    securing for limited Times to Authors and Inventors the
    exclusive Right to their respective Writings 
    and Discoveries;

> let's just tax the hell out of AI companies and redistribute.

That's not what I favor because you are inserting a middleman, the Government, into the mix. The Government ALWAYS wants to maximize tax collections AND fully utilize its budget. There is no concept of "savings" in any Government anywhere in the World. And Government spending is ALWAYS wasteful. Tenders floated by Government will ALWAYS go to companies that have senators/ministers/prime ministers/presidents/kings etc as shareholders. In other words, the tax money collected will be redistributed again amongst the top 500 companies. There is no trickle down. Which is why agreements need to be between creators and those who are enjoying fruits of the creation. What have Governments ever created except for laws that stifle innovation/progress every single time?