The weirdest collector I’ve ever had personal knowledge of was a guy my mom knew who collected cannons. Not cannon replicas, but actual cannons. He told my mom one day that he’d only marry a girl if she collected cannons as well, and he eventually met a nice cannon-collecting lady, and they indeed got married.
To answer the OP’s question about what drives people to collect something odd, here’s my take on at least some collectors: I’ve noticed a lot of the hardcore collectors in my various hobbies take on a self-given status as “historians” or “archivists,” and believe that their collections, due to their breadth and depth, are saving for posterity some otherwise overlooked aspect of the hobby. And, for the most part, they’re right, but the collecting then becomes self-perpetuating; you get more fire hydrants or cannons or whatever, become more aware of minor variants and such, and seek to fill in THOSE gaps in your collection, ad infinitum.