After a certain time, people stop counting days and count in weeks, then they count in months and then years.
For the same reason, if you meet someone after a while, you recollect how many months ago you met them before or years.
The brain doesn't calculate the days passed if you say '4 weeks ago'. But if you say '29 days ago', the brain needs to calculate to relate to how many weeks is that. Or similarly, '52 weeks ago' or 1 year ago.
Since we have all been taught to calculate with aspects of the larger format in order of Seconds, Minutes, Hours, Days, Weeks, Months, Years, Centuries and Decades, it is second nature to us. We remember 1 month = 30 days quicker than we would remember how many months 52 days would be.
While minutes under an hour are still okay, but when it crosses an hour mark, using minutes is meaningless since it increases the Cognitive Load on the user.
A lot of websites might even provide the exact date if you click on the '1 Week Ago' representation
With regards to Just Now, it also doesn't make sense to show seconds which increment every second. For example, 20 seconds ago. It will then need to be dynamic and counting forward. So, a good solution would be to have it as Just Now and make things simpler. After a couple of minutes, one could continue using the minutes passed format.