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The reasons are logical, as well as the chinful front aspect being the iMac's trademark for the best part of two decades, but as others have said, it's a desktop machine, thin & light have little to no relevance here.
The chin on this new thing looks particularly incongruous without an Apple logo. Wasted opportunity to really push the design of an all-in-one desktop in my opinion, with the result looking gawky & Fisher Price.
the chin has the 'computer' bits vs the last gen where they were behind the display