That is because >50% of internet traffic is bots. Mostly BS marketing tools, ad conversion scams and quack-security 'scans' from my experience. Pretty sure the entire captcha proliferation is to reduce fake clicks in google ads.

I've been working on ways to block bots from some websites I manage the hosting for. It is nearly impossible. I rate limit the hell out of most of the world and block a long list of fake user agents. Still like 40% of out traffic is bots.

>'legally challenge CAPTCHAs'

Probably not, depending on where you live etc. Here, in the netherlands only gov. websites are required to be accessible.