Sight reading is a skill that one develops naturally when one is taught phonics. No one literally sounds out words like "the, an, copy" etc. when they have seen these words thousands of times over their lifetimes. But what happens when the reader encounters a word he/she has never seen before? If, for example, I use the word "tetrachlorodibenzo-p-dioxin" that you have never seen before, and you have only been taught sight reading, how would you even know what the first phoneme is, much less figure out how to pronounce that word? But if you know phonics, you can figure out how to say that word and come close to the actual pronunciation if you don't get it exactly.
Those who teach sight reading are jumping ahead to the natural outcome of teaching phonetically and skipping all of the steps in between. It is analogous to teaching kids multiplication and division without bothering to teach them the processes that multiplication and division are built on--addition and subtraction.
By having studied LATIN.
Latin is the foundation of the romance languages. Not only does studying Latin give you the ability to pronounce the word, but to understand exactly what it means.