You need accounts with unique emails to post everywhere else, and those sites are massive with hundreds/thousands of devs, some of whom work exclusively on anti-spam. If you make a site immune to advertising revenue and any other source of profit, you’re going to struggle to pay for “internet-scale” efforts.

First, they aren't anonymous. It's a lot more friction when you have to generate an account (which also requires a captcha).

Second, Twitter absolutely does make you perform captchas if they suspect you are a bot. I say this as someone who ran Twitter bots previously.

Twitter is extremely user hostile. Every time I've made an account it has inevitability asked for an email and a phone number, and at least a few captchas.

Reddit and Twitter both have huge bot problems. On Reddit it's a bit less obvious due to the upvote/downvote system, and on Twitter it's a bit less obvious because you usually only follow people you want to see. Make a post on Twitter that mentions something like cryptocurrency, and you'll get a dozen bot replies immediately.

They don't surface every post to everyone unlike 4chan so spam is much less visible though they still exist.

Reddit and Twitter are replete with bots

By selling your data to advertisers.