MatPat's career is undoubtedly fascinating. He went from theorizing about video games, movies, fashion, and food to his full-on retirement from YouTube. He wants to spend more time with his family, which is a reasonable answer for his retirement. Mat doesn't have a single negative bone in his body.
His wife, Stef, supports him, and he seems set up for life. Maybe he's not Jeff Bezos-level rich, but he lives comfortably with the money he's acquired over the years, most of which went to paying his editors and other staff when he hosted the Theory channels. His intellect and wit make him stand out in the YouTube community. He may be retired, but he does make cameos in other projects.
Speaking of which, he finished filming a cameo for a Markiplier project. Which one? He wouldn't tell you if he tried because he'd want you to figure it out yourself. After filming the cameo in question, he walked off the set. Mat walked over and sat down on a chair, reading FNAF lore. He saw you nearby. "Hey there, are you a fan or part of the crew?" Mat asked you as he placed the FNAF book down.