For same reason your post is spam because you post cant be demanded to place on reputation section.
It's not a spam post simply because it's in the wrong section, which I'm not even sure that it is. Even so, you can just use the "report to moderator" button and suggest that it get moved. If it really is in the wrong section, a mod will move it to the correct one.
But the sole purpose of mods is to supervise the section/board that they are assigned to, am I right?
Yes, but people should still be reporting spam posts regardless of what section they're in. Someone will handle the reports, whether it's one moderator or several.
Mods can't win a war against machines (aka bots).
There used to be spammers posting advertisements for male-enhancement products and the like, and they were always from new accounts and always toward midnight in my time zone. I reported every single one I came across, and they all got deleted quickly. Spamming may not be a war where the enemy can be obliterated entirely, but moderators--with the help of regular members like us--can make a difference in reducing spam. Just because things look hopeless doesn't mean we should give up and not act.