When we're considering 'Line Warfare' there are several factors that need to be accounted for when we explain the tactics of the time. These are the presence of cavalry, the difficulty of communication, and the characteristics of infantry weaponry. I'll deal with these in reverse order.

To begin with: pretty much every infantryman on an 18th (or early 19th) century battlefield will be using a muzzle-loading smoothbore black-powder flintlock musket. Here's one being fired. There are several things to note. First, the loading procedure is long and involved, and if just one step was done incorrectly, the musket could fail to fire. Second, our firer manages three shots in 46 seconds, which was very quick by contemporary standards, and probably would only have been sustainable in the first minute or so of a firefight. Three shots a minute was considered an acceptable standard for professional troops, four or five the mark of elite troops. After a minute or two of engagement, most infantry battalions would manage one or two shots a minute. Third, observe the amount of smoke the musket makes, or watch this. Now multiply that by five hundred and we can see the amount of smoke produced by a battalion volley. Fourth, muskets were fairly inaccurate; the maximum effective range of a smoothbore musket is generally given as around 100 yards, and the maximum accurate range as something like seventy or sixty yards. Close-up volleys could be seriously damaging to enemy battalions, but longer-range musketry was unlikely to break a unit or inflict significant casualties.

All of this together means that a single infantryman could produce a very small amount of fire at fairly close ranges with limited visibility - certainly close enough ranges that charges, whether by cavalry or with bayonets, were an integral part of military tactics. Most armies viewed an infantry firefight as almost an antecedent to a bayonet charge, with the firepower of a battalion 'softening up' a target so a charge could drive them off. However, delivering or resisting a charge requires a formed body of men - if every soldier is spread out in a skirmish chain, then each soldier is able to make the decision of whether to stand or flee on their own, and since they lack the reassuring presence of a formed body of friendly soldiers, they're more likely to break and run in the face of a charge.

This brings us on to the next point: communication. This is an era before the invention of radios or telephones, and the amount of smoke present on a battlefield made visual communications impractical. That meant the only real method of communications available to battalion commanders were shouted orders or musical commands, via a drummer or trumpeter. This obviously required every soldier in a battalion to be in hearing range of the officer or drummer, meaning that formed bodies of men were the only real way for commanders to exercise command and control.

This point is especially significant if we consider how prevalent cavalry were on an 18th or early 19th century battlefield. As I mentioned before, charges were very practical - a cavalry charge could cross the effective range of an infantry battalion quickly enough that the infantry could only fire one or two volleys at them, and given the poor visibility of the battlefield it was very possible for well-led cavalry to surprise infantry - and if infantry were still in line when cavalry charged home, then the unit's ability to resist was essentially nil (this is especially true of skirmish lines, which were almost universally destroyed by cavalry charges unless the infantry could reach friendly battalions or ground that cavalry could not easily move through).

The only practical method for infantry to resist cavalry was a hollow square formation (although Austrian forces in the later Napoleonic Wars experimented with a 'mass' formation that was improvised at the battle of Wagram). Forming square took time, however, and careful, precise movements of men, in order to present an unbroken hedge of bayonets which would dissuade enemy cavalry (and, more importantly, their horses), from charging home. A spread-out skirmish line could not form square in enough time to stand off cavalry, but a unit in a simple line could.

There were troops who did not fight in line - light infantry especially. These, however, were elite units - highly motivated, trained, and with enough discipline to retreat and re-form as necessary, and they were still a supplement to line infantry, cavalry, and artillery, really until the widespread deployment of breech-loading rifles in the latter half of the 19th century.

So, in conclusion, line tactics were not suicidal - in fact, due to the conditions of an 18th-century battlefield, not adopting them would have been suicidal.

Sources:

The Art of Warfare in the Age of Napoleon by Gunther E. Rothenberg

Swords Around a Throne: Napoleon's Grande Armée by John R. Elting

The Bayonets Of The Republic: Motivation And Tactics In The Army Of Revolutionary France by John Lynn

Fusiliers by Mark Urban

Rifles: Six Years with Wellington's legendary sharpshooters by Mark Urban

Redcoat: The British Soldier in the Age of Horse and Musket by Richard Holmes