A single beam column with the machine around it costs millions already, and takes quite some space. Wafers are simply not large enough to accommodate more than, I'd guess, two or three columns at the same time. Each with independent optics.

A somewhat simple 2x2 cm Si photonics Chip in my line of work takes about 24h exposure for two layers - a full scale wafer is hundreds of times larger, more complex, and has dozens of layers. The math, physics, and geometry just don't really work out (yet)