Ain't trenches dug by common soldiers of regiments and engineers only do 'refined' jobs?
WW2 US Army and USMC infantry are instructed how to dig foxholes individually.
Between a foxhole and complete entrenchments and Field Fortifications there is a considerable difference.
A foxhole, as the name implies, is a single hole for one or more soldiers. It is not connected to anything else and unless improved, provides no overhead cover against air bursts, plunging mortar fire, or direct fire from heavy caliber weapons that can penetrate a few feet of dirt (which is anything heavier than a 12.7mm machinegun). One man with a personal entrenching tool can dig a foxhole in less than an hour.
A "full profile" trench system (trenches you can stand up in and still be covered) with underground bunkers to protect the squads and teams, covered positions for heavy weapons, fully camouflaged against air and ground observation, is much more complex. The Soviet army in WWII, probably one of the best forces in the world at 'digging in', calculated that a rifle regiment and all its heavy weapons (up to 120mm mortars and 76mm howitzers) could be completely fortified with trenches and covered positions and camouflage in 24 hours. Adding minefields, barbed wire, and other obstacles would take much longer, but as the Germans facing them quickly realized, any Soviet rifle unit given a day to dig in could only be dislodged by a full-scale attack with artillery support and, ideally, tanks - nothing less would do anything but pile up casualties. Ironically, the modern Russian army is relearning this lesson the hard way against Ukrainians who have apparently read the Soviet 1942 Infantry Manual (which, Full Disclosure, I translated into English over 20 years ago, so it's not as if it were a secret!)
The trench warfare of the Western Front in WWI was a peculiar situation, caused by both sides having far too many troops for the frontage, so that there simply was no room to maneuver, and enough troops to not only dig in completely but also have multiple lines and zones of defensive works stretching for kilometers behind the 'front line'. The result was really more like Siege Warfare than any maneuver battle, and since Civ has never really modeled siege warfare very well, it is hard to replicate "All Quiet on the Hexagonal Early Modern Front" in the game.
What is needed in Civ VII is to take a lesson from
Humankind and separate regular Field Battles from Siege Battles so that they are not fought and resolved the same way - and then also admit that when armies are numbered in the hundreds of thousands or millions, they may devolve into a Siege situation by digging in and forcing the adoption of Siege techniques to dig them out again.