1/ Because they are locked in an arms race. Hackers innovate new ways around Twitter/Facebook anti-bot measures as fast they they develop new anti-bot measures.
2/ I don't know about you, but I frequently fail at answering the CAPTCHA designed to detect if I'm human. I fail to identify all the traffic lights or buses in that picture. I really don't know what letter is a U, u, v, V, or whatever.
3/ In any case, you can go all "mechanical turk" on the problem and hire low-waged workers in foreign countries to create the accounts before you then have software programmatically post to those accounts.
4/ Ultimately, all the questions "Why can't those evil Facebooks/Twitters do this one simple thing?" comes down to "Because the problem is not as simple as the ignorant pretend".
