Well it has happened - just not with home computers like the ZX Spectrum or Commodore 64, per se.

I'm thinking specifically of the Atari Flashback 2, which was a single-chip reimplementation of the Atari 2600 (even has cartridge connector solder points on the board). Then there's also the innumerable NES, Super NES, and Sega Megadrive/Genesis clones that have been produced recently...