Why isn't it required that both presidential nominees announce their VPs at the same time?

Question

Why isn't it required that both presidential nominees announce their VPs at the same time?

Answer 1

They should have rallies, speeches, and visit the same places at the same times right?

Answer 2

Because it is their decision to make and there is no dead line. McCain may not have picked Palin as soon as Obama picked Biden.  

Answer 3

The constitution sets the rules for electing the President and Vice-President.  That topic isn't covered.

Answer 4

Maybe you'd like this same rule to pick who would be President too.. save a lot of trouble from those pesky voters