Why do you spell thieves like that and chiefs with a normal plural?

The plural thieves is an exception, since the other nouns belief, brief, chief, and relief all form normal plurals with an S. But note that the verb forms believes and relievesare like the verb "thieves" (robs).

The adoption of Words into English from other languages creates various pronunciations, and the spelling usually reflects that. But there seems to be no reason why the form "thiefs" is not used.

(The Word "thief" comes from the Germanic theof so we cannot blame the French.)