The simple answer is historical accuracy. The infantry - all armies - moved around the battlefield at a slow marching pace of around 70 to 90 steps per minute, with their muskets/firelocks carried more or less vertically at the shoulder. It was done that way for very good reasons: it was essential to maintain the integrity of the firing lines (by maintaining shoulder contact) while moving, also the pose with the musket at low port (or 'charge bayonets') is very tiring with a ten pound plus, five foot long weapon with a 15 inch bayonet on the end. Trust me, I've tried it. The musket was only brought down to eye level for the last few paces of a bayonet charge. And for Hollywood movies.

The plethora of marching figures is long overdue IMHO and very welcome; perhaps one day the same approach will be taken with cavalry sets instead of all the headlong galloping Derby racers waving swords aimlessly all over the place!