[StavrosK]

Because there may be a request in the system already for the price you offered, which means that your trade will go through immediately when it gets offered, and you won't be able to take it back, even in microseconds.

If any amount of time passes, it's considered to just "be sitting there", so you can change your mind and cancel it. Microseconds isn't the same as no time at all, when you can pre-issue buy or sell orders that will get filled at the very instant a matching order goes in.