Why do websites prevent pasting via onpaste="return false;"

Why do websites do this? Isn't it trivial for bots etc to easily side-step any protection this might give? Or is it just to make people "double-check" their inputs? I find it completely maddening because I would assume I'm way more likely to correctly input a copy-pasted e.g. routing or account number than if I have to manually enter anything. Can we just agree this is terrible UI and is probably detrimental to correct data entry? What am I missing? In frustration I even sometimes open the dev tools and remove this attribute on elements out of spite.