It's actually for security. Otherwise websites could grab whatever you happen to have in your clipboard. You might have a copied password from a password manager, or a private key for a wallet. I don't want sites having defacto access to the clipboard for that reason.
Surprisingly enough, given the extensive backstory of the botnet / Alphabet Inc. / Google being fucking arseholes, this is not the result of arsehole design. It's just that websites don't have access to your clipboard.
A few notes:
It's actually for security. Otherwise websites could grab whatever you happen to have in your clipboard. You might have a copied password from a password manager, or a private key for a wallet. I don't want sites having defacto access to the clipboard for that reason.