The reason? You can't imagine how big the facilities for building car components are. There are billions and billions invested on facilities with decades to be paid off.
This is a big advantage once cars are standard, and all the same,and don't change, but becomes a huge liability if you change the technology.
If you invest on a plant a billion dollars to build engines to be amortized in 25 years and engines become obsolete by the way... you have thrown to the trash 1.000 million dollars as your expectations for the future became wrong.
Now multiply this billion for 20 or so and there you have the car industry. They don't want change because change means them losing billions.
Then there are other minor issues: electric means no mechanical reviews which is a huge business. Electric means mechanical engineers without work.
Any change has to come from newcomers and the same way Elon did: You do your sales yourself so car manufacturers-dealers(who profit any time a mechanical piece fails) do not sabotage you.
I don't see any conventional car company doing it, maybe Toyota that advanced what the future would wring before the others that are late to the party.