Sure, you'll always have more stuff to dig out.
But you won't always have all that you want/need right now.
Hence, some mods require lots and lots of resources, or some really rare resources that only generate once ever very few chunks.
Infinite resources in the ground does not mean unlimited availability, if you only have a few hours a week for play.
===
A race that can utilize the power of stars, and travel from star to star, will "never" run out of power. They can still be short of what they need right now.
I like this. It's poetic and explains a lot of why we like MC.
Direct answer to the most direct question of the OP (the title):
We have power systems because they are centralized, which increases efficiency. Imagine having, instead of a "power system," just furnaces. You set up hoppers, and whatever sorting system, and you put coal in each of them. But the
issue is that you can't be sure if the amount of power in each machine is more or less even. The most basic system (using "at a crossroads, split in half" logic) would put 64 coal in the first furnace, 32 in the next, then 16, 8, 4, 2, 1, then none in any of the others. Obviously, you can improve this with redstone logic and stuff - but the amount of effort it takes to get even distribution is disproportionately large. Power systems rectify that, allowing us to put power in every machine in the line in a fairly simple fashion. Also, from a mostly lore based standpoint, electricity is more efficient than coal - in can be turned on and off; if I'm smelting, say, 9 items, I'll use one coal's worth of power for the first 8. Without electricity, the last would use ANOTHER coal - with electricity, I use 8 + 1/8 of a coal. The efficiency gain seems minuscule, but it's nice because of large scale projects and OCD (not necessarily in that order
).
On a slightly unrelated note, I'd like to (for the sake of discussion) define a "magic" mod and a "tech" mod. Theme is too shallow - let's only use theme to define a mod as a last resort.
My personal definition is this:
A tech mod focuses on the centralized power system mentioned above, as well as automation. This would make Buildcraft a tech mod - the focus is on automating the movement of items and fluids, and later tier stuff requires power. Thermal Expansion, Minefactory Reloaded, Mekanism, Rotarycraft, and others (as well as all their addons) fall under this definition.
A magic mod has a more fuzzy definition - the most concrete thing I can find is that they focus on the improvement of the
player themselves instead of on improving automated systems. They also
don't focus on power systems or automation. Although many, if not most, magic mods do have some form of both, they are often underdeveloped, or have niche uses, and you could play through the early and sometimes mid game just fine without them. These things manifest themselves primarily as wearable or hand-usable tools, weapons, or armor, often manually crafted at intricate multiblock structures. So, Thaumcraft would be a magic mod, as would Ars Magica 2. Blood Magic and Botania both blur the line; they have centralized power systems, but they also focus on manually made tools
for the player. Theme-wise, they're magic, so as a last resort I'd call them magic (keep their fuzziness in mind, though).
