Ah, so the original PC did simple parity checking only. And I believe that the PC AT did the same (definitely not less). However, very many PC clones right from the beginning never did even simple parity checking (and often had no DMA either, which the PC and PC AT had), and that set the trend toward no checking at all in the whole realm of consumer computers. By the time laptops came along, this was the norm, and only a few really high-end laptops ever did even simple parity checking (let alone ECC).
By the way, ECC in the PC realm uses the same 9-bit RAM as for simple parity checking, but uses the parity bits over a whole block of bytes to do ECC. Doing ECC on a per byte basis would require 11-bit RAM.