Kev,
Probably the same reason that sebonac only gets one star, the greatness of Old MacDonald is muted by the restrictions associated with Doak having to accept the input of others.
I'm sure there is some truth to that. On both courses, there are a couple of things that I wasn't 100% happy with, but deferred to others; and when you are voting on the top ten courses of the last 15-20 years, it doesn't take many of those to put one out of the top ten.
On the other hand, I had no qualms about Barnbougle Dunes or St. Andrews Beach, which are also collaborations [with Mike Clayton].
In the end, Sebonack is just too hard for my tastes, and Old Macdonald, I just haven't played enough to be comfortable putting in the top ten. [I have not been back to play since opening day.]
I would think most people would be put off by how many courses of my own I've rated highly, instead of thinking I'm selling myself short. The only two courses where my own ratings might be above the consensus are Rock Creek, which I feel strongly about, and Cape Kidnappers, which I think some people sell short because they enjoy the views and miss the architecture, and because there is a general bias in favor of links courses.