While I cannot vouch for all the towers, the ones I am familiar with are all located in strategic lookout positions at the top of cliffs overlooking critical passageways. They were thus better suited as a reinforced early warning system with a modest amount of firepower available for rapid intervention or to provide supporting fire against a more coherent assault than as the principal underpinnings of a comprehensive defense.

For instance, 2 of the Martellos in Québec City overlook the St-Laurence seaway above the narrows in front of Lévis, while a 3rd one overlooks the densely populated basin of the St-Charles river lowlands where a hostile landing party would have done considerable damage in a short span of time. In each case, the towers stand atop nearly vertical cliff faces about 100 meters tall. There used to be a 4th one, but I’m not sure where it used to be. Thus, they are positioned as reinforced outposts which complement the Vauban-designed fortifications of the Citadel in the upper town which protects the urban core.

Perhaps they were deployed differently elsewhere, but in this case they were clearly meant to play an auxiliary role and provide early observational capabilities with other reinforced positions providing the bulk of the defensive capabilites.

Martello towers featured artillery. The elevated height of the tower allows guns to shoot further, the fortifications protected the garrison against attack, and they were often positioned at strategic points, such as sea points, chokepoints, or in tandem with another. In addition to being able to shell incoming enemies, they also provided intelligence: namely, if the enemy went around the towers as you suggest, that would reduce their possible location to be outside of the range of the towers, which makes it far easier to determine where the enemy is, and by extension, his/her next course of action, objectives, size, and so forth.

Source: Fort Frederick, Canada (museum which is itself a Martello Tower)

it wouldn't be a huge project to divert an army and supply lines around it

As a general rule, it is a huge project to make an army of any size do anything whatsoever, especially if it involves roads. Clauzewitz observes that "in war everything is very simple, but the simplest thing is difficult". Have you ever moved a regiment - several thousand men, multiple guns each weighing many tons, hundreds of wagons, thousands of livestock - down a side road to get onto a course parallel to the original one, without getting lost, tangled up with the three other regiments trying to do the same thing, or into a huge traffic jam? Just telling that many people what to do is a huge undertaking; you know there's going to be several someones who don't get the word, or don't get it right, and blunder off the wrong way and get killed by wandering into the main enemy force. Or, if it's that kind of army, "don't get the word" and get into a convenient position for deserting, or "getting lost" and showing up a couple of days later, after the battle, full of excuses and loot.

In short, forcing an enemy to do anything they wouldn't usually do, like divert away from the best road through an area, is itself a valuable contribution to a defense.