The encoder's embedding is contextual, it depends on all the tokens. If you pull out the embedding layer from a decoder only model then that is a fixed embedding where each token's representation doesn't depend on the other tokens in the sequence. The bi-directionality is also important for getting a proper representation of the sequence, though you can train decoder only models to emit a single embedding vector once they have processed the whole sequence left to right.
Fundamentally it's basically a difference between bidirectional attention in the encoder and a triangular (or "causal") attention mask in the decoder.
Is there a difference here other than encoder-only transformers being bidirectional and their primary output (rather than a byproduct) are input embeddings? Is there a reason other than that bidirectionality that we use specific encoder-only embedding models instead of just cutting and pasting a decoder-only model's embedding phase?