There isn't a delta because E=mc^2 isn't just describing a reaction, or a conversion. It is describing the fundamental equivalence between mass and energy. It is true even in situations where ΔE and Δm aren't defined.
"It is describing the fundamental equivalence between mass and energy" I agree that this is widely agreed upon when referring to E=mc^2. It's almost a dogma by this point. But the derivation Einstein used to come up with his equation doesn't actually support said dogma. I'd really like to understand how this "fundamental equivalence" came about and the proof behind it, or if it's just dogma. E=mc^2 is a lot more profound than E=delta m * c^2. IIRC Einstein in the final sentence of his paper tries to generalize his result and jumps on the E=mc^2 train without backing it up.
I think you could say Einstein proposed the fundamental equivalence between mass and energy as a hypothesis and there has been much evidence since. Apparently it was based partly on a proposed symmetry between space and time. Einstein being Einstein probably had some deep reasoning behind it which was not necessarily all put down in that paper.