What are some practical reasons why some countrys still use the imperial system?

Answer 1

The cost to redo the infrastructure is prohibitive. Think of all the things around you that would have to be changed. Currently in places like the US, the entire manufacturing process would have to be changed. Parts re-sized, every piece of packaging reworked. While there are many areas where changes have taken place, such as some beverages (you buy 2-liter bottles rather than half gallons of soda). Almost every mileage marker has to be removed, reset and remade. It would double the number of tools needed for many years, double the parts for everything from cars to dishwashers to every other consumer good. It is the overlap period and initial change cost that prevents it happening in most places.

And there is a historic/traditional aspect to it as well.

The scientific community has pretty much standardized on the metric system on a global basis.