The Confederacy did try large-scale use of irregulars. The Partisan Ranger Act of 1862 was the legal justification for this. The guerrillas played a forgotten role in the shift of the Union to the hard war approach, as Union soldiers gradually lost sympathy for Southern civilians when they got shot at and then everybody played innocent about it. The thing is that irregulars aren't necessarily useful in all situations or a panacea to every single strategic dilemma.
As a side note, Arkansas was the first place where the Confederates sanctioned the formation of irregular guerillas bands. That sanction was withdrawn after the Confederates realized that many irregular bands turned bandit very quickly. To my knowledge, none of the subsequent confederate guerilla groups in Missouri, Kentucky, TN, WV etc. were officially santioned. That is not to say that CSA units did not cooperate with the irregulars, especially during large scale raids (Morgan), but the various groups were not part of the official CSA order of battle and their actions were not coordinated by CSA officers.
The Federals did make guerilla groups, including some with nasty reputations part of their official order of battle. For example, many of the "non deployable" unionist regiments from east TN were guerillas and opportunistic bandits who had been given blue uniforms and a regimental number. Like their confederate counterparts, the unionist guerillas / regiments tended to alienate even nominally pro union civilian populations as the sometimes fuzzy line between military unit and bandit group got crossed.