Because the Georgia state Republicans are doing their damnedest to gum up the system.  Here's what will happen:

1) During and immediately after election day, conservative outlets will report unfounded rumors of voter fraud in heavily Democrat Fulton County.   Fulton County went 70/30 for Biden in 2020.  
2) Soon thereafter the Republican-controlled state election board will express concern about the rumors of fraud and investigate the claims.  
3) Conservative outlets will whip up their followers with more and more stories about nonexistent voter fraud.  
4) The state election board will claim that the rumors require that they take over counting of the ballots in Fulton.  
5) Due to this investigation Fulton county will not finalize the count in time.  This is 10% of the entire electorate in Georgia and about 15% of the Democrat vote.   
6) With Fulton County out of the total, The Georgia Secretary of State will either certify the results for Trump or not certify the results at all.  
7) If the Georgia Secretary of State does certify for Harris, the Republicans will immediately sue in federal court to rescind the certification citing the rumors and investigations of voter fraud with the goal of pushing the decision to the Supreme Court like in 2000.  

If you don't think this is exactly what is about to happen, consider this: the Georgia state Republicans created the election oversight board to implement this plan after Secretary of State Raffensperger refused to throw the election to Trump in 2020.  The five board members are currently three Republicans, one Democrat, and one Independent.  The board has been implementing a number of last minute rules such as giving county officials wide latitude to delay certification to investigate allegations of fraud.  The one Democrat board member has been sounding the alarm bell for weeks now about how the conservative majority is operating.  

It going to be a wild 10 weeks.  As a Fulton County resident, I feel like a hostage of the state Republicans.