I mean they don't just target Pikachu. Team Rocket deciding to steal someone else's pokemon for an episode is common as dirt, and their methods vary from the low budget to the expensive to the deceptive and its a hole or a scam Ash will fall for it every time, Pikachu is just the default target. Ash is kind of the one that needs some help if he keeps falling for Team Rocket in bad disguises trying to sell them a scam service for the literal hundredth time.