Why? Most of them skip the full ruleset.
ZX Chess implements chess rules except for castling, promotion, and en passant.
Those are major rules! You could forgive ZX Chess in 1983 but modern tiny chess variants should not "cheat" by skipping rules.
I'd love to see a list of tiny chess programs that fully implement chess rules as defined by FIDE .
Even better if these programs implement UCI - https://en.wikipedia.org/wiki/Universal_Chess_Interface
There's nothing impressive about a supposedly smaller program that has access to unlimited memory, unlimited screen memory, lots of BIOS routines, et cetera, and STILL can't honor the rules.
One-upping a ZX81 in terms of sheer number of bytes of code is not impressive, and it wasn't at all what 1K Chess was about.