What is this racetrack-shaped Special Use Airspace, and why isn’t it on the VFR sectional?
This is indeed a designated air refueling track used by military tanker aircraft (all AR-nnn SUA areas are, and they all share that same general racetrack shape).

The reason why it isn't on a sectional is because it's simply not a concern to anyone who isn't in a high-performance, high-altitude airplane -- air refueling (of fixed-wing birds at least) typically takes place well into the FLs (FL280-FL330 is typical, although it may be lower or higher depending on what the tanker is gassing up), and doesn't bother anybody below.