Because they're muppets. Barclays used to do this with me, one of the reasons I no longer bank with them.
The solution is, if they're offering something you need, get a phone number from them and call them back, checking first to ensure they've given you a number recognised as belonging to that company. If it's a sales call, tell them you can't give out that information for data protection reasons and enjoy the bemused response on the other end of the line.
