Because PANSOPS says so. Outbound timing commences when passing abeam the holding fix or when reaching wings level, whichever occurs later.
What PANS-OPS section 6 says is:
1.3.4 Start of outbound timing
Outbound timing begins over or abeam the fix, whichever occurs later. If the abeam position cannot be determined, start timing when the turn to outbound is completed.
Not quite sure what "over the fix" would mean, but it seems to make clear that you should start timing when you know you're abeam, wings level or not.