Recordings can only be set from the live point

If I come across a show in the guide that is of interest but already 1/2 over I can press the start over button.

I can also watch again from the beginning up to 24 hours (or more) after it is aired. Then on demand for some. 

Why then is it not possible to record the entire show from the beginning, but only from the live part?

If I go back to start over I should be able to press record.