There are at least two major reasons for this. First – in 18th century, cavalry was still a thing on the battlefield. Single soldiers spread out on the battlefield would be perfect targets for it, while a larger formation had a bit of a better chance at defending itself. The other is the accuracy of the firearms at the time – it could not reliably hit targets, so you needed a volume of coordinated fire instead, hence a bigger unit to provide it.
Bear in mind that at the same time, part of the infantry (light infantry) did operate in spread out, skirmish formation, partially for the reason you are mentioning. They often used rifled carbines, which were more expensive but also more accurate.
Latest Answers