Why does the decoder only transformer models predict the logits for each token in the prompt instead of just give the entire prompt as input and then start predicting the next token?

For example:
Suppose I have this text as input: "my name"
Why does the gpt do a forward pass for each token ( it would start by token and then try to predict my then do the same for name etc).

*Why does it try to predict an information that it already has?*

Why not pass "my name" as input and only then try to predict the next token?

decoder example gpt languagetechnology next prompt text the decoder the prompt token transformer transformer models