Nice question. You will likely (and I hope you do) get more a sophisticated answer than mine, but I expect evaporation is one.

Consider a marine/brackish tank where salinity increases as evaporation proceeds. Same deal I believe in freshwater where among others Ca2+ density would increase.

Maintaining stable osmoregularity is important for fish health, moreso than stable pH per se, I believe. And WC help maintain that stability.

A completely different answer is that some do in fact report having quite excellent results (meaning stable, healthy system) with "low tech" tanks in which water changes are quite infrequent, as implied by your question.