All ballots must be received and accounted for BY midnight election night. This doesn't mean that all votes must be tabulated by election night, but there can't just be 100k random ballots appearing out of nowhere. Imagine what it means when 100k ballots show up two days after deadline - where were these being kept and why? That's why mail-in ballots are so difficult because they allow for back-dating which means they don't have to be registered and accounted for by the deadline. They also, obviously, won't show up in time for the deadline.
It's the equivalent of mailing in your homework assignment on the day it's due instead of physically turning it in - you know there is no way in hell your assignment is getting to the professor in time. What would your professor say if you walked into class without your assignment and told him "hey it's all good I mailed it to you - should arrive in about a week".
All ballots must be received and accounted for BY midnight election night. This doesn't mean that all votes must be tabulated by election night, but there can't just be 100k random ballots appearing out of nowhere. Imagine what it means when 100k ballots show up two days after deadline - where were these being kept and why? That's why mail-in ballots are so difficult because they allow for back-dating which means they don't have to be registered and accounted for by the deadline. They also, obviously, won't show up in time for the deadline.
It's the equivalent of mailing in your homework assignment on the day it's due instead of physically turning it in - you know there is no way in hell your assignment is getting to the professor in time. What would your professor say if you walked into class without your assignment and told him "hey it's all good I mailed it to you - should arrive in about a week".