Why aren't PCB mounted male mini USB connector parts common?
This is because Mini USB has been essentially phased out as depreciated, the newer standard is the USB micro-B bus, which is compatible with 3 times as many sockets as a mini USB. 

The mini USB never caught on, not only because of plug, but because of its size. If one were to use a mini USB, the height was very similar to the USB A plug, so those were used instead.

Male SM components were exceptionally uncommon, as there are few uses for devices which have such small hard points. The smaller ports would often break and bend easily compared to the large ports. On top of that, smaller USB form factors were designed specifically for a device that would need a small socket due to space constraints (EG: phone, console controller). If someone needed a Device that plugs into another, there wasn't really any reason that A couldn't be used.

However, There are still many mini A-type USB male SM components out there, I have a sneaking suspicion you're looking on common shopping websites, as opposed to component distributors, the largest known to sell to consumers are Mouser and Digikey.