Why does ATX power supply claim 60a at 12v when it only has two 18awg 12v wires?
Because there are many more than just two cables. By my count that PSU has the following for 12V:


24pin ATX - 2 cables
2x 8pin PCIe - 6 cables
1x 8pin EPS - 4 cables
3x Peripherals - 3 cables


That gives at least 15 cables - or at 7A per cable, 105A cable capacity.

The point is, that total load is expected to be distributed between different connectors, not just through one of them. Each of those different connectors have a specific power rating - with the EPS connector being the highest.

Modern CPUs are power hungry so the EPS connector with 4 cables became the modern standard having been expanded from the earlier P4 connector which only had 2 cables for 12V to the CPU (and going even further back, just the 20pin ATX connector powered the whole motherboard).

Modern GPUs are even more power hungry, so multiple cables are chained together - the high end boards have both an 8-pin and 6-pin connector which allows 6 cables in parallel.