The reason may not be as rational as you might assume.
According to Merriam-webster.com:
DID YOU KNOW?
If you find yourself tempted to spell today’s word “firey,” you’re relying on sound logic. “Fiery” was formed by combining the word “fire” and the “-y” suffix, so it is reasonable to expect that the result would be spelled “firey.” At the time that the adjective was coined in the 13th century, however, the spelling of the noun had not yet become standardized. One alternate spelling was “fier.” Presumably, it was this spelling that eventually led to English speakers settling on “fiery,” even as the lone surviving spelling of the noun turned out to be “fire.”