Molly mentions he has very warped sense of time, and he does not seem to be able to realistically perceive how many years have passed since the last time he and Molly have seen Lìr, until it's too late. He himself says that he is not too willing to use magic on himself, even if for healing purposes, because it "dulls" the feeling of being mortal and human.

But most importantly he probably has an underlying, paralyzing fear of losing his loved ones. Schmendrick is the one who is stubbornly clinging to the idea of younger, heroic Lìr, despite all evidence that poor Lìr is old and withering (on the subject, I wanted to mention a lot more things but I want to avoid spoilers, even though it's very telling how he reacts and what he does towards the end of the story). Just imagine how he would react at the slightest sign that Molly is unwell, or if she ever happens to experience a health scare or an injury (the tree that fell on her that she mentions, anyone?). The thought of outliving someone he loves and cares about, after who knows how many years of not being able to form life-long connections because those around you will invariably die of old age while you are stuck in time, would probably send him tailspinning in a panic.