Because they suck?
There is this cashline service FOC with a UOB credit card too, which I don't use, which they send me a statement just for it that says $0.00 every month.
So one day I had a chance to call their customer hotline, and ask if that was really necessary to send me a blank statement month after month despite expressing the fact I don't use or need it. To which they replied its FOC, goddamnit I don't need them to send me a blank statement every month to know its FOC.
At least AMEX is wise enough, they let me choose between paper and online, or have both, and no they don't send me rubbish.
They need to check if there is any linked payment to other accounts. Who know if some1 has transferred the money to this account, you have transferred a sum of money to another, or you have unsettled bills? A sum of unaccounted money fly to nowhere when there is broken transaction.
Bank can close any account immediately.
But ...read what Lokey has posted.
they will use that time and call you
and say that they will offer you a ferrari pen or a travel bag
if you contnue to keep the account
("no no you don't even have to use it if you don't need and u won't get charged")