[Espressosaurus]

Programs run on an operating system, the operating system runs on real hardware.

The real hardware gets old, wears out, parts become difficult and perhaps even impossible to source.

The operating system accumulates known vulnerabilities until it's no longer safe to connect to anything.

You can work around the latter two problems with emulation, but it's never the same--display technology if nothing else is different and presents differently. Emulation is dependent on the fidelity of the emulation. It's much harder to make it exactly cycle-and-timing accurate, though in most cases (like Word 2007) it doesn't matter.

The instructions might exist, but they are not runnable without other supporting infrastructure.

This also ignores programs that are wholly reliant on third party compute and instructions you have no access to that can be shut down and no longer available, like your MMOs.

[mikewarot]

The IBM 1401 is still being emulated to this day. It will never die.

I sometimes emulate a DEC VAX 11/780 running OpenVMS 7.3 on my phone.