Why doesn't Frank Oz perform with the Muppets anymore?

Background
Staring in the mid-1990s, Frank Oz began to transition himself away from Muppet performing to focus on his up and coming directing career.

In a 2007 interview, Oz explained why he distanced himself from the Muppets: "There were a lot of reasons. One was that I was a dad, I have four kids. The reason was that I was constantly asked to do stuff. And also, I had done this for 30 years, and I had never wanted to be a puppeteer in the first place. I wanted to be a journalist, and really what I wanted to do was direct theatre and direct movies. So it was a slow progression, working with Jim, but I felt limited. As an actor and a performer, you feel limited because you're not the source for the creation, and I wanted to be the source. I wanted to be the guy and show my view of the world. And if I screw it up, then I screw it up, but at least I tried.  And as a director, what you're really showing is you're showing the audience your view of the world. I don't know why, but I thought I say things a certain way, and I wanted to express myself. I've always enjoyed, more than anything else, bringing things to life, whether it be characters or actors in a scene or moments in movies. I've done so much with puppets, that I've wanted to work with actors."

During the filming of Muppet Treasure Island, Oz was not available since he was directing Bowfinger at the time, so Kevin Clash performed Oz's characters on set, and Oz looped the dialogue in post-production.

During filming for Muppets from Space, Peter Linz performed Oz's characters on set, and again Oz looped the dialogue in post-production.

Sesame Street puppeteer Eric Jacobson starting performing Bert in 1997 on a semi-regular basis, and in 2002 he started performing Grover as well, due to his strong puppeteer talent and extremely similar vocal-range. (Veteran puppeteer David Rudman took over the role of Cookie Monster in 2001).

In 2000, Oz retired from performing Miss Piggy, Fozzie Bear, Animal, and Sam the Eagle. In 2001, Jacobson secretly debuted as Miss Piggy's new performer in a video greeting during Muppetfest, and made his first major Muppet production debut performing Fozzie Bear, Miss Piggy, and Animal for the 2002 TV movie It's A Very Merry Muppet Christmas Movie. He took over the role of Sam the Eagle starting in 2005 with The Muppets' Wizard of Oz.

Although Oz occasionally still performs on Sesame Street (at least once a season), today Jacobson is the principal performer of Bert and Grover, as well as the principal performer of Muppet Show characters Miss Piggy, Fozzie Bear, Animal, and Sam the Eagle.

The Muppets
Evidently, Frank Oz was offered the chance to perform in The Muppets, despite the fact that as noted, he had retired from Muppet performing. In Fall 2011, when speaking of the early script of the then upcoming film, The Muppets, Oz was quoted in a UK article as saying "I turned it down, I wasn’t happy with the script. I don’t think they respected the characters. But I don’t want to go on about it like a sourpuss and hurt the movie.""

This quote was soon highly publicized, picked by several U.S. Internet articles and blogs, combined with un-sourced claims that some of the film's Muppet performers were unhappy with the movie. Oz's Muppet retirement in 2000 was a relatively quiet one, leading readers to believe that his dislike of the early script was the sole reason for ceasing to perform with the Muppets. Although his true departure from the Muppets was over ten years ago.