We have never had all votes counted in two days.
Military absentee votes do not even all have to arrive until 8 days AFTER an election.
Absentee Ballots rejected, have a week to be cured by the voter in some states.
Provisional ballots are also given a week to be verified that the voter is legit, and had not voted already by any other means, before they are counted....
Etc etc etc
If you have a close race, which we've had many lately as a near evenly divided nation, you can not call an election on election night or even in a couple of days, because all votes would need to be counted.....including the Military vote due 1 week after election night.
On election night, most swing states only have election day, machine votes counted, then you have early voting to count, then absentee voting to security check and process then count their votes, then there are votes that arrive on election day in polling place ballot boxes to count, provisional to count, military to count etc etc etc
We have the safest and most secure and most accurate elections in the world, which also favors the citizen's right to vote and pick their representatives and leaders.