If VC's don't give founders a "no" (in case the situation changes, the startup takes off) then why would you as an employee?
Say you have 30 offers on the table. What's to be had by saying "no" to 29 of them instead of just "yes" to 1 and not replying to the rest?
If that 1 drops out you now have 29 pending job offers.
If you "declined" the rest, you now have what pending?