My guess is that what Mike is really referring to when lamenting not writing with Brian on the reunion album is that there are no “Wilson/Love” collaborations.
You make a very good point. Yet in 2006 when the guys got the platinum (?) records for "Sounds of Summer" on the Capitol Tower rooftop Brian reportedly gave Mike a demo of a new song and asked if Mike wanted to write lyrics and the latter declined -
https://youtu.be/OTSeL27Wq00?t=126 It sounds like this could've been a Wilson/Love tune. Now, I don't know what Mike's reason is. I have my theory but it's just that.
Ah, I had forgotten about that bit from 2006. Good catch.
I think we can all deduce the likely list of reasons Mike didn't want to work on the song Brian suggested:
1. It may have been an existing co-write with another writer, and Mike clearly isn't into piggybacking onto songs Brian already wrote with others
2. It may have been something unlike what Mike was/is into, sort of like how Mike reacted to the sending suite on TWGMTR by pretending to shoot himself in the head in the Rolling Stone piece.
3. Maybe Mike just thought the song sucked. We'd obviously have to hear it to know. I don't think it's impossible that Brian might have tossed something towards Mike that was rather slight. But Mike I think would tend to like the slight stuff more than stuff like "Lucky Old Sun" material or "Pacific Coast Highway."
4. Related to some of the above, but somewhat distinct, is that I think Mike maybe wouldn't have wanted a collaboration to have a "Solo Brian" vibe, and in general he wasn't as into just guesting on each other's solo stuff.
5. 2006 was still a somewhat politically fraught time in the Beach Boys World. I think the Al lawsuit stuff had just very recently been wrapped up, and I think the "Smile" lawsuit stuff from Mike had only recently been shot down. I'm not sure what was still pending in various suits as of that date in 2006 (I think some of the remnants of the Smile suit may have still been lingering, but I'd have to re-check dates), but even if the slate was clear, the lawsuits were still pretty fresh at that point. It had been very recent that one of Mike's lawsuits had alleged Brian threatened to pull Mike's license and go out with Al as "The Beach Boys", and it was only months later at the end of 2006 that Brian did start doing some gigs with Al for the first time (though that all crashed and burned by mid-2007).
I think 2006 was always going to be a rough time to try a BB reunion, even if just Brian and Mike trying to write something. By 2011/2012, the lawsuits had been over for longer, I think Brian had kind of run through so much solo stuff that he was ready for something else, Al had been ready to do a reunion for years, and even Mike seemed to recognize the 50th would be a point to do *something*.
I think by the end of C50 in 2012, they all learned that a "reunion" could work, but that it looked and felt different than what some of them (Mike perhaps) might have thought or assumed or wanted. It's unfortunate that he used that information to reaffirm his thinking that he should just tour on his own, because I think they could have taken the information from the 2012 reunion to *refine* how to continue doing it in a way that would work and minimize the beefs.