Planed obsolescence may be part of it but modern cars are electricity hungry so it's not surprising batteries don't last like they used to.
Our family car when I was growing up was a locally built 1967 Chrysler VC Valiant (Dodge 106).
It had the classic slant 6 225 cu in engine and the original battery in that lasted until it was sold in 1987.
20 years from an original battery???
There was lots of room in the engine bay and it was a BIG BLACK chunk of plastic.
But that car only had basic electrics and no power windows. The radio was just AM with a single speaker.
An old friend had the V8 Lux version with electric everything and he was lucky to get 3 years from a battery.