Why are there not any aftermarket bolts if these things snap so easily?
I haven't had any of these oil filter cover screws* break, and that's after numerous filter changes. As for those who have had them break, no doubt it is due to applying too much torque. In some cases owners were using a 3/8" drive torque wrench, which is way too large for this application. Hard to believe, but I recall reading a post here of at least one owner using a 1/2" drive torque wrench.
One thing to keep in mind regarding these filter covers screws is that you really wouldn't want a screw that is of a higher, stronger grade in this application, because if it were to be over tightened you'd end up with damaged threads inside the crankcase cover. Better to have a lower grade fastener that will break when over tightened well before thread damage occurs.
As it has been said over and over in this thread and others, these screws do not require very much force in order to compress the ridge of the metal cover gasket sufficiently to prevent oil from leaking around filter cover.
* Also worth noting is that they are not really "bolts"... rather they are considered screws. Honda's parts list diagrams refer to them as screws. I'm not aware of a hard and fast definition of what is considered a screw vs. a bolt, but from what I've seen it seems that in terms of thread size anything 8mm and smaller is considered to be a screw. Whereas a fastener with a 10mm thread or larger would be considered a bolt.