Why shouldn't the amount of Stimulus package be announced earlier?

I do not know is there a precedence to pre-announce the amount of stimulus package in any countries, but isn't it good to tell everyone that the Government is planning to spend XYZ amount to help the country going thru the hardship, instead of just keeping it like the underwear:

You see it, but I did not expose it.

You see it, but you don't see it all!