Or they don't know how to effectively use the API, maybe.
The Twint README calls out reasons for going beyond the API at the start - things like Twitter's increasingly strict rate limits and the limit of only 3,200 historic tweets for a user account.
Believe me, I would LOVE if the official API supported threads. I have tried several times to make it work, but the official APIs are stuck in a circa-2012 idea of how Twitter works. Replies just aren't a thing to it.
Most scrapers including this one lack these characteristics.