There are too many bots out there that are very inconsiderate and do not limit or throttle themselves.
We have one right now that crawls every single webpage (and we have 10's of thousands) every couple days, without any throttle or limit. It's likely somebody's toy scraper, and currently it's doing no harm, but not everyone has the server resources we have.
The point is - if you are dealing with inconsiderate bots, a captcha of some type is pretty nearly a bullet proof way to stop them.
With that said, Cloudflare usually is smart enough to detect unusual patterns, and present a challenge to only those who they believe are bots or up to no good. If every person gets a challenge, then the website operator is either experiencing an active attack, or has accidentally set their security configuration too high.