The reason is because to do a proper 800 workout your legs need to be springy. It's possible to run high mileage and have fresh legs for workouts but you need to do your easy runs as "true jogs" (7:30 min/mile or slower). To train optimally for 10k you should be running your easy runs fast enough to put some stress on your system, which means your legs will always be a little tired, which is ok since it is possible to grind through distance workout days on tired legs.