Guerrilla tactics are only useful if you don't mind your country being occupied in the mean time.

Given that many southern states cited the protection of the institution of slavery as a key cause for the war, being occupied by northern troops wasn't an option as that would have allowed them to destroy the very thing they were fighting for.

Putting large armies into the field to resist occupation was the south's only option.

Guerrilla tactics were not necessarily less costly than conventional tactics (I'm not sure what you mean by that phrase), and were much less successful at actually winning battles against conventional forces. Even in the French and Indian War, the pivotal battles were not skirmishing actions; they were sieges, or conventional battles, like the Battle of the Plains of Abraham, which resulted in the capture of Quebec.

Additionally, Civil War commanders tended to look back not to the French and Indian War or American Revolution, but to the Napoleonic Wars. Skirmishers and light troops were an important component of a Napoleonic-era army, but could not win battles on their own. Infantry in loose order were very vulnerable to cavalry, and were much harder to communicate with for the purposes of command. Prior to the widespread adoption of rifled firearms, irregular shooting was also less effective than a musket volley in breaking or driving off enemy infantry. Some of these factors were beginning to change, with improvements in technology, but military thought--as learned by the participants in the Civil War, hadn't quite adapted. Civil War armies did include light troops, and infantry (or cavalry) would use skirmishing tactics where appropriate--but that wasn't everywhere.

Interestingly, our term 'guerrilla warfare' comes from the uprising against Napoleon's occupation of Spain. The Spanish guerrillas successfully made Spain a terrible ulcer for Napoleon, and helped prevent Napoleon's generals from solidifying control of the country, but notably, the French were only driven out of Spain with the conventional forces of the British, Spanish, and Portuguese under Wellington.