And why should the words "crisp" and "crispy" both exist?

Yes, but both crispness and crispiness exist, too. (Crisp is also a Latin loanword.) Why do people feel that honesty somehow has an injunction against its cousin honestness, which breaks none of the morphological rules of English? Is it because the French brought us honnêteté (from the earlier Latin honestas?). And why is honestly (mixing Latin root and Saxon suffix) perfectly OK? (Did honestment just not make it over the channel?) And closely related, there's honor, honorable, and honorableness.