They don’t because it would be against their financial interests to do so.
Advertising, blue checks, admitting that they have fewer genuine monthly active users than they currently report all negatively affect their bottom line. And any moderation solution that could work would be decried as censorship and people who do genuinely engage could leave the platform.
If you were Twitter/X, how would you improve their current (probably state of the art) "malicious-content" bot detection + mitigation?
It sounds like it ain't just captchas and IP banning?