Convenience.
All foods have phosphate, it is a building block.
If you want to do the best for your fish then live foods like live worms, fresh frozen clam (the kind you would buy live to eat), quality frozen like LRS, and dried (not roasted) nori. You can add some selcon or fish oil too.
I completely disagree pellets are better then what I just listed.
Dry is a packaged processed food and not fresh food. What do we eat to be healthy? Fresh food not processed foods. The key is to pick the correct frozen foods as not all are equal.
Just look at the health of Paul B's fish