It has been ATX12V/EPS12V since around the Pentium 4, which itself only added more 12V rails to the original ATX connector to supply more power. Moreover, ATX a

It has been ATX12V/EPS12V since around the Pentium 4, which itself only added more 12V rails to the original ATX connector to supply more power. Moreover, ATX and ATX12V power supplies have the same dimensions, use the same mounting, and the original 20-pin ATX connector is still the same. The newer power supplies can generally be used on older system boards going back to the mid 1990s and many of them separate the 24-pin connector into 20+4 for exactly that reason. It's not just the case and monitor. Drive bays have standard sizes and mounting (2.5", 3.5", 5.25"), the SATA connector has been the same since its introduction even though modern systems support higher speeds using it (and is also electrically compatible with the SAS connector and SAS HBAs typically support both), expansion cards have standard dimensions and use standard PCIe slots, etc. They'll make breaking changes from time to time (e.g. PATA to SATA, PCI to PCIe), but it's typically a decade or more between them and then many systems will support both for years after that. PCIe was introduced in 2003 but you can still find new systems with 32-bit PCI slots, and SATA to PATA adapters are available. If you want to you can build a PC with the latest CPU, install a SCSI HBA into it and connect that to a hard drive from the 1980s that will fit into the same drive bay as a modern 3.5" hard drive. Granted there are more space constraints in a phone, but that doesn't mean you can't do it at all. And an iMac or Mac Mini is not ATX. There is no law requiring every device to follow every part of the standard if it's necessary to satisfy some other constraint. But a G5 iMac is basically slag; the contemporaneous ATX PC has half a dozen components that could plausibly be reused in a modern PC.