Your post seem to still assume that "rationalism" actually has something to do with seeking truth, rather than with seeking, for example, self congratulation about truth.

To give a concrete example, there's a lot of truths to learn in physics.

A lot of those truths are in the physics textbooks and the rest require very hard work to figure out. So, when someone's interested in knowing truths that have to do with physics, they learn the math, they study physics, they end up able to answer questions from the homework section of a textbook, and sometimes, even able to answer the unanswered questions. You get a fairly reasonable island of knowledge, not a protruding rock of "MWI is true" in the vast sea of near total ignorance.