Trenches were much, much more complicated than a simple linear line. Even so, a battle in a trench is still a battle, the attackers may have an advantage due to surprise, initiative, and special weapons but the trench is still full of armed enemy soldiers, there's no guarantee here. More to the point, trenches typically zig zagged, which gives an inherent defensive advantage around corners. This was for multiple purposes. For one, if an artillery shell lands in a long linear trench then it can do a maximum amount of damage, flinging shrapnel out across the whole line and hitting a huge numbers of soldiers. If the trench has zig zags then an artillery shell will mostly only damage its individual "zag". This also, of course, applies to lines of fire. You don't want enemy soldiers to be able to drop into the end of a long trench line, set up a machine gun, and be able to cover the whole stretch with a minimal number of men.

But, of course, if you want to defend a trench you will want to put in plenty of fixed machine gun emplacements which cover parts of the trench to protect against it being taken. Which was exactly what was done. Within the trench system there would be lots of strong points where defenders could fire their rifles or fixed machine guns on attackers who had taken a section of trench. Additionally there were bunkers and even fortresses within the trench system, which were much more difficult to take.

Taking a section of enemy trench was incredibly dangerous. You and your squad are now cutoff from support from your own forces, while your enemy has a supply line that basically runs right up to the back of the nearest enemy soldier to you. This is why it was so difficult to move the front line on the Western Front, you needed to take a big chunk of enemy trench all at once for it to actually be a feasible endeavor, otherwise your cutoff group of soldiers will just fall victim to a counterattack, but doing that would take a tremendous amount of resources, careful coordination between different groups (combined arms tactics), and special weapons, something that only developed near the end of the war. Prior to that the tactic of choice (other than massive ill-fated frontal assaults) was trench raiding, sending a group of highly trained and specially equipped soldiers to attack and take a trench, complete an objective (destroy enemy equipment, kill enemy troops, collect intelligence, etc.) then abandon the trench before a counterattack and return.

So there's a few things to unpack here.

Firstly, while trench systems on the western front were extensive, they weren't all interconnected.

Secondly, trench systems were, especially as the war progressed, composed of multiple lines of trenches - a front line and then additional support lines, redoubts and communications trenches running perpendicular to the front. Early in the war it was common to have a single front line trench that was densely packed with troops. It was quickly learned that this arrangement was quite brittle and exposed troops to artillery fire. So as the war progressed trench systems became more and more elaborate.

Later in the war, while trenches continued to exist, a much deeper system of defense developed that was composed of connected trench lines, but also concrete bunkers and other types of redoubts. Frequently the "front line" was lightly held with additional troops positioned to the rear and able to move to reinforce threatened sections of the line. Machine guns were positioned in concrete bunkers as well as in shell holes in order to create large zones that could be swept by fire. The idea was to create a "defense in depth" that would expose attackers to continuous fire as they tried to advance. Rather than having a single trench line or even a series of trench lines separated by only a few hundred meters, the "zone of battle" was a deep network of defenses that could stretch backwards a half mile or more.

This sort of defense in depth was designed to bog down attackers, reduce the number of defenders needed overall and provide time for the defenders to counterattack. While the French and British employed defense in depth, the Germans, who mostly stood on the defense, created the most elaborate systems of defense.