The difference is that you can use new parts in computers from 2010s. You can also replace them easily without much difficulty, as the standards haven't really changed that much.

But computers from the 80s and 90s are not compatible with modern platforms. Standards have changed, and new hardware thar uses standards like 32-bit PCI, ISA, MCA (for expansion cards), IDE are no longer manufactured. Even the CPU architecture had big changes between early x86 CPUs.