Does it cost them extra money a month to have the items available for people to purchase?

Does it cost players anything if they're not interested?

Do people like to complain about optional items that are only there for people who actually want it?

They're completely optional. They don't impact the game/make it pay to win. It gets SE a bit more money when people purchase the items to help them potentially upgrade stuff on their side and improve the game.

Sorry you think that the optional items are like sandpaper and it's rubbing you the wrong way~