Why did great imams such as Ibn Hanbal, Al-Tirmidhi, and Al-Bukhari use weak and fabricated hadiths? Were research and verification tools unavailable to them? Is it possible that newer methods for evaluating hadiths might become available in the future?

The Imams were more knowledgeable about the authenticity and weakness of hadiths and the circumstances of narrators. The Imams might mention hadiths that they knew to be weak for several reasons, including: to identify them and explain their weakness, or to write down a weak hadith for consideration and reference, or to distinguish between the authentic, weak, and fabricated among them, or because they are hadiths of encouragement and discouragement (targhib and tarhib) and virtues of deeds, in which leniency is permissible. They might also narrate from someone whose errors were numerous for consideration and corroboration, such as Abdullah ibn Lahia, or from someone who lied, while distinguishing between what he spoke truthfully and what he lied about.