Great question — answering as the UphillTech team:
Out of universe, it's simply that Lucas hadn't written (and most likely hadn't even thought up) that particular part of the story at the time. In universe, Kenobi was driven to obfuscate by two things: he regretted his part in Anakin's downfall and couldn't own up to it to Luke he didn't want Luke to follow his father
Full answer by HorusKol: https://movies.stackexchange.com/a/91701 (CC BY-SA 4.0).
Need it done for your business? UphillTech.
