Why does the iMac not allow memory parity error checking?
This is not Apple specific at all. First, why is ECC (72-bit) used over parity? The reason is simply that DRAM chips have been made in data widths of 4, 8 or 16 bits for decades. ECC lets you simply add another commodity chip, instead of making a specific low-volume 5 bit chip. Besides, when you have parity, you can't correct any error, so all you can do is crash the computer when an error is detected. So the net effect is to cause more computer crashes, even if the error is in part of the RAM that is not in use.

There's two reasons why Apple can't put ECC in. One is simply the integrated memory controller Intel puts in does not support ECC to force workstation/server makers to buy the more expensive Xeon line. You can check the ECC supported processors on the Intel ARK.  You see it's only Xeons and low-end CPUs (server appliance market), but none in the Core i5 or i7 range.

Second, the SO-DIMM pinout simply lacks data pins for the 72 bit bus in order to shrink the size for laptops. Memory bus routing is very difficult and the extra 8 lines would also make it difficult for laptop motherboard manufacturers.

Many numerical simulations do not require ECC. Real-world tests have suggested that the extra cost and speed penalty of ECC isn't worth it. As another example, some companies even tell you to turn off ECC.