Touch is a moment in time
Even though the spell says the casting time is 12 hours, the "touch" component is just that, a quick touch. Let's look at other spells that have a non-Action casting time but require touch.
Awaken: 8 hour casting time
After spending the casting time tracing magical pathways within a precious gemstone, you touch a Huge or smaller beast or plant.
Here it is quite clear that you first spend the 8 hours in prep work, then there is a momentary touch.
Create Homunculus: 1 hour casting time
While speaking an intricate incantation, you cut yourself with a jewel-encrusted dagger, taking 2d4 piercing damage that can’t be reduced in any way. You then drip your blood on the spell’s other components and touch them, transforming them into a special construct called a homunculus.
Once again, there are a number of actions that do not require you to be in contact with all the of the spell components. Only at the end do you finally touch the completed mixture.
Glyph of Warding: 1 hour casting time
When you cast this spell, you inscribe a glyph that later unleashes a magical effect. You inscribe it either on a surface (such as a table or a section of floor or wall) or within an object that can be closed (such as a book, a scroll, or a treasure chest) to conceal the glyph.
This one is not as straightforward, but if we take it to mean that the touch much be for the entire hour, that means you cannot remove your writing utensil the entire time. You would have to create your glyph as an etch-a-sketch. So I would still go with the definition of a definitive touch last to activate.
Create Magen: 1 hour casting time
While casting the spell, you place a vial of quicksilver in the chest of a life-sized human doll stuffed with ash or dust. You then stitch up the doll and drip your blood on it. At the end of the casting, you tap the doll with a crystal rod, transforming it into a magen clothed in whatever the doll was wearing.
This one really pushes the point home, but since it's only available to people that bought the book I saved it for last. There are a LOT of motions going on with a doll, a vial, stitching, and then finally a tap (touch) to finish the spell.
From those examples (and more I didn't include), it's clear that a range of Touch only means that there needs to be physical contact at the end of the casting, not the whole casting time.
So let's look at Simulacrum again
You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell.
So the subject needs to be within touching distance the entire time. But the actual touch happens at the end. You spend the eight hours sizing up the subject and building a snowman (snowwoman, snowbeast), but you don't have to hold their hand the whole time.