It's more about the accumulation of fecal matter over time. I don't feel convinced that a washer removes it all, hence staining. And I don't feel great about making my kid sit in another kid's shit.
There are many decisions young parents make that from the remove of a keyboard in a happily single or child-free relationship seem irrational or (mon dieu!) _inefficient_, but there is a emotional depth to these choices that are _very_ meaningful to the people involved.