Using a normal usb charger to charge MacBook Pro, 1m type c cable is ok but 2m is not, why?

"Thicker wires means higher conductance and lower resistance per unit length"

Only if the alloy composition of the two conductors are perfectly identical. Cheaper cords can, and so do. come with lower grade alloys. Some I've seen looked to be Aluminum, not copper.

Not necessarily. The thickness should be selected based the length. Thicker wires means higher conductance and lower resistance per unit length such that a longer cable can be the same resistance as a shorter one. But it depends on the design.

Often passive USB extension cables don’t work because the cables aren’t oversized.

A two meter cable's added resistance attenuates the electrical values just enough that they don't meet the Mac's retirements.

I've noticed this too, and that's the best I could figure out. Now I stick with Apple accessories ... even cables.

Your basic problem with using a USB-A to USB-C cable is the power limit since it will only operate at 5V. I've tried that before with various Macs, and in System Report it always says it's maxed out at 15W, which is 5V x 3A (the current limit for most USB-C cables).

You should consider getting the Apple AC Cord (aks Power Extension Cable) instead which will give you a longer reach from the electrical outlet to the charging brick. Here is the link to the US version which is also grounded, but each region has their own version:

Power Adapter Extension Cable - Apple