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I think it's a combination of both...
The aluminum body is stronger than the glass, but still very thin, the larger the glass is (with no pillars between) then the more reinforcement and weight necessary around the window frame.
Modern glass technology makes it easier, but airplanes are slow to change. It was harder to make an "any shape you want" glass in the 60's. Newer airplanes can and do have more expansive single windshields alot of the time that i've seen.
The more appropriate answer would be I think "they had to up until the 60's" after which the auto industry in particular started pushing the envelope. If you can show alot of airplanes both designed and built after the 60's (aircraft like the Piper Cherokee were first made in 1960 and are still being manufactured TODAY, they wont change a design because things like FAA certification have to be redone for even small changes) I might scratch my head too, the above is a guess but probably a good one.
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