Why didn't Sheikh al-Kulayni directly narrate from Imam Mehdi (AS)?
Sheikh al-Kulayni (250-329) lived during minor occultation of Imam Mehdi (عليه السلام). He could have directly narrated ahadith from Imam (عليه السلام) through his (عليه السلام) deputies making shorter reliable asnaad. Why did he took pains to meet narrators who could narrate ahadith of Imam Ja'far (عليه السلام) through longer asnaad (which in many cases are unreliable)?
Had he consulted the Imam of time (عليه السلام), at least shia community would have a book that would have been near to 1
Salam your definition is based on Sunni definition of Daheef (weak)
Shia definition
https://fa.wikishia.net/view/حدیث_ضعیف
Ṣaḥīḥ Ḥadīth (Arabic: الحديث الصحيح, lit: Authentic Hadith) is a hadith which its chain of narrators would be connected to Imams (a) through Shi'a and trusted hadith narrators.
https://en.wikishia.net/view/Sahih_(Hadith)
Reliable Hadith Al-ḥadīth al-muwaththaq (Arabic: الحديث الموثق ) or reliable hadith is a hadith with a full continuous chain