When Amorraim spoke they weren’t speaking off the top of the head. They weren’t sharing theie musings. Rebbe Yochanan says in Shabbos about the Pasuk, Emor Lachachma Achosi At, that you may only say something when it is as clear to you as the Issur of marrying a sister. Rebbe Yochanan also famously scolded a Talmid for having a hard time believing a Drush that he derived from a Pasuk.
Mikra Ani Doresh is the final word. Rebbe Meir almost had Rebbe Gamliel removed from his post with a Drasha from a Pasuk.