The Quantization of Electromganetic Energy

Isn't it strange that there is no theoretical proof till now of the quantization of electromganetic energy ? The Planck hypothesis was accepted since it yielded the correct spectral distribution of the blackbody radiation energy. Poincare showed that quantization is a necessary and a sufficient condition to obtain the correct blackbody formula. In QED, the field is quantized in an analogous way to the quantum harmonic oscillator, it is not a still a theoretical proof for quantization. In quantum optics, many experiments verify the quantization hypothesis, but still no theoretical proof !!