Typically the teams you see are being split between several projects and one construction project mnanager is managing several projects.
Then throw in city permitting and inspections which take an awful amount of time and expense.
Is there a reason it's slow? Yes, they're trying to keep budgets low. However there isn't a reason it needs to be slow.
When there's a political will and cultural to do it fast and safely, it gets done. That bridge collapse in Philly on i95 took like 10 days to repair. It also probably cost a lot to repair as well but there was a political will to do it because of how critical that infrastructure is.
Same goes for bridges and highways on California destroyed by earthquakes or landslides. Those get rebuilt quickly but the cost to do so is very high.
There is also real physical limitations when moving dirt around. If you quickly make a big pile it settle quite a bit. So instead you add a few inches at a time and bring out heavy equipment then let it settle, do some tests, and repeat.