One of the most common tips for creative writing is: "Is this the most interesting part of your character's life? If not, why aren't you showing us that?"
Post-MGS3, the Big Boss games have all been fairly pointless, self-contained side stories. Peace Walker had to invent a reason for its own existence (the ridiculous notion that Big Boss spent the last ten years not believing or not understanding The Boss' final sacrifice, or whatever bullshit that game was trying to sell me), and for all that talk about the big heelturn, MGSV is basically just Big Medic killing the mean man who blew him up in Ground Zeroes.
Meanwhile, we're missing out on seeing Big Boss and Solid Snake's mentor/student bond, or Big Boss starting his own sovereign nation, or his decision to hold the world at ransom with nuclear weapons. His years with Zero, Eva, Ocelot, Sigint and Para Medic, starting The Patriots? Nope, we don't get to see any of that. All of the major events in his life are delivered to us through second-hand accounts in Cassette Tapes, or the fucking timelines in the end credits.