Headset spacer hate

Wife displeased with the stack I gave her when re-jigging her fucking cantis following Space Horse purchase

You could flip the stem and lose most of that spacer stack. Having a scoblestack of spacers under a negative angle stem is like walking from Harrods to Selfridges via Piccadilly Circus

I just wondered if there was a technical reason not to use a single large one as opposed to 4 smaller ones.

It's purely a commercial imperative - stocking a few sizes and adding them together to make the required size is much cheaper than stocking spacers in 2mm increments from 2mm to 50mm - that's 25 SKUs compared with 2 SKUs to stock 10mm and 2mm and sell them in batches.

There is a rue of thumb (not mine) that for CF steerers the total spacers should not exceed the diameter of the steerer

There is no rule of thumb, there is a set of specific fork manufacturer guide lines, usually in the region of 50mm maximum spacer stack under the stem.