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You can't cut sharp inside corners using a cnc mill (round bits), as one would want for tabbed fixtures.

Most sheet good are too large to be placed on or moved around by a cnc mill table.

CNC milling is a slow process (unless you can drop six figures on one).

CNC mills take a lot longer to setup and program, and also require more skill to have a successful result (no chatter, not breaking bits, ramping into the cut on interior features).

Good CNC mills that can handle steel are massively heavy and large machines which makes transport and setup difficult for the home machinist.