Notice that three times the "Mc" is changed to "Mac".
I had a Scots friend who told me it was a social error
to confuse them. The Scots insist on "Mac".

There's also, M' or Mag. Anyway, it's an orthographic convention and as such probably is not much subject to reason. The real names would've been written in their Gaelic forms: e.g., macDhomhnaill for MacDonald or Macdonell (as in the famous Sanskritist, Arthur).

Aha! Thanks, jheem--now when any of us get called down for spelling, we can just say, "Sorry if you don't like it, but it's an orthographic convention".

My Grandmother used to put a little squiggle under the c of Mc to indicate the missing a. Personally I think the abreviation squiggle would take as long to write as the a but who am I to argue logic against orthographic convention.
I always wondered if some Mac's became Mc's through non-squigglement.