I've wondered for quite some time why not triangles or hexagons. I guess the yield (percentage of wafer thrown away) improvements would be minimal. Plus, the temperature is often better controlled at the center, which would make the edge parts less performant anyway.
That's the other advantages of chiplet design: maximize yield (a small defect renders a much smaller chip unusable), and much more granular binning (easier to sort out good/worse chips, due to placement and random issues during fabrication). Not to mention you have a much more modular design at the end, where you only have to change the cheaper (not 7nm) silicon interposer.
Partly it's "path dependency" - everything is set up for rectangular dies, so everything would need to change for uncertain benefits. Not just the tooling, but also the design software. While I was looking at this I found that Intel has a patent for an octagonal die with a smaller square one fitted in the gaps between: https://patents.google.com/patent/US20060278956
BTW the dicing used (in the 1970s) to be done partly by hand. You can see a video of someone doing it here: https://youtu.be/HW5Fvk8FNOQ?t=978
That's the other advantages of chiplet design: maximize yield (a small defect renders a much smaller chip unusable), and much more granular binning (easier to sort out good/worse chips, due to placement and random issues during fabrication). Not to mention you have a much more modular design at the end, where you only have to change the cheaper (not 7nm) silicon interposer.