Well when you have each state that can have its own election laws, then yeah of course you are going to have manipulations and inconsistencies. This is what happened at the 2020 election. I mean it's a federal election, WHY would you not have election laws to be from a federal level, implemented the same across the entire country? I mean if a ballot is received after the election day, then it shouldn't be counted. If it's allowed to be counted in Texas, then it shouldn't be allowed to be counted in Michigan either. Why would you allow each state to come up with its own rules to allow one thing in one state and disallow the same thing in another state? Does not make sense.
The federation, contrary to popular belief, is actually contingent on the consent of the states. Part of that is that the states get to decide various things. It's messy, but in the end I prefer it.