Because their scope is bigger than anything else. If they did what you wanted and worked on all their IP at once, you'd hate it because it wouldn't in any way resemble a Bethesda game. It takes their whole studio committed completely to it for half a decade for their games to exist.
They absolutely are only actively developing one game a time. They might have a handful of teams that don't have a specific project for the game they're actively developing doing rough broad strokes on the next project, but that next project absolutely isn't being developed.