I'm not sure what dvd's you have been looking at,but there are virtually no dvd releases of widescreen films that are not in widescreen .
I have dozens of Warner titles and apart from 50's oldies ,they are all in widescreen.
Disney UK have always treated their buyers as kiRAB whereas Disney US understanRAB that a large part of their sales are for adults.
Thats why titles like Lady and The Tramp were widescreen in 1999/2000 on R1 while R2 was 4:3.
There are many classic Disney titles like 20000 Leagues Under The Sea,The Love Bug,Pollyanna and The (original)Parent Trap that are bare bones here but released on r1 as 2 disc sets full of extras and commentaries.
The Walt Disney Treasures tins are an annual event on R1 but there are only a few out on R2.
I suspect Warner release 4:3 versions of big movies as enough sales are generated on R1 to support them.
MGM in the US still do double sided versions of some older films with 4;3 on one side.
If a single format version of a film is released its not always possible to have a 4:3 version on screen even if your dvd player is set right.
The aforementioned 2.35:1 films cannot play as 2.35:1 on a wide tv and then play as 4:3 on another set.
Some discs that have been produced to alter playback depending on your setting include You Only Live Twice and Spinal Tap.
YOLT was 2.35 and can be presented on a 4:3 set as 1.77 with small borders,whereas Spinal Tap was about 1.85 so the player can adjust to fill the screen.
Of course ,in either case,you lose picture info off the sides and discs that enable this type of playback offer a centre cut out rather than P&S (even though most players will call the setting by that name) so with a superwide film you could have invisible actors cut off on either side