The Metal Gear Solid games span across decades, though not in chronological order. This should clear the timeline up.
After the release of Metal Gear Solid 4 in 2008, Konami wanted to ease their development process for future games. They ...
That led to the creation of the Fox Engine that powered Metal Gear Solid V: The Phantom Pain. In the meantime ... introducing characters and themes that would be central to the game.