Seems odd to be fighting a standard war against a mroe industrialized foe with more manpower, an overall superior enemy by any definition, except perhaps quality of generals. But that only increases the case for guerilla tactics as Union commanders like good old Daniel Sickles would get himself ambushed a million times over.
In the case of Spain, Germany and Italy were spamming the hell out of expeditionary corps and sending Franco his aid while Negrin got squat and only had a ragtag militia army. No rationale to not use guerilla warfare there either.
First of all, in many cases southern forces did fight like guerrillas. Especially notable are Confederate cavalry units under commanders such as Jeb Stuart and John Hunt Morgan, who used hit and run tactics to harass (and in some cases completely immobilize) Union armies. In addition, Union-occupied territories such as Missouri, Kansas, and western Virginia were constantly under threat of "bushwhackers" (essentially terrorists) that attacked both military and pro-Union civilian forces.
However, I assume you're talking about the larger armies, especially earlier in the war. Some Southern leaders, especially president Jefferson Davis, did initially want to fight like that. From our old friend Battle Cry of Freedom:
Jefferson Davis agreed; early in the war he seems to have envisaged a strategy like that of George Washington in the Revolution. Washington traded space for time; he retreated when necessary...he tried to avoid full-scale battles that would have risked annihilation of his army and defeat of his cause...
However, non-military issues prevented him from implementing this policy:
The second factor...was the temperament of the southern people. Believing that they could whip any number of Yankees, many southerners scorned the notion of "sitting down and waiting" for the Federals to attack. "The idea of waiting for blows, instead of inflicting them, is altogether unsuited to the genius of our people..."
Even at the end of the war, Jefferson Davis encouraged Confederate armies to fight on as a guerrilla army. However, Robert E. Lee was opposed, and his surrender (along with the capture of Davis) discouraged widespread adoption of this strategy.
TL;DR: Southern forces did use guerrilla tactics at times, and wanted to do more, but both civilian and military pressures forced them to use a more traditional strategy for much of the war.
A few other notes.
While resorting to a guerrilla war could certainly have prolonged hostilities, consider that right at the outset what sort of message that would send to the populace.
"Hey sorry but we aren't really going to try to defend your towns or cities, we are going to camp out in the woods and wait till the Union troops show up, hope they don't loot or steal, or rape too much in the meantime"
Is that a force that you are really going to support? Certainly some will, and some will be pushed to it, but that is already from the get go ignoring the possibility for a traditional victory on the battlefield to spare occupation.
The Civil War was also not one all together for the Union, concerned with winning the heart's and minds of the South. it was focused on forcing them to their knees first, and then in the spirit of Lincoln extending the hand, but the corrective hand had to be used first.
So if it had been guerilla from the get go, instead of battles of maneuver like Shiloh or Gettysburg, events like the March to the Sea, or the Carolina's could easily have become the norm. The war would have been even more violent and hard on the South.
A poor battlefield general can still be an incredibly hard occupier, and compel subjugation if given sufficient time and resources, and can sway previously disenfranchised populations (gee wonder where they could have found those in the CSA) to build sources of loyalty, as Butler did in New Orleans.