Because most users of games don't really care for this. They are either too young, or to busy with other facets of life that they just want to play the game.

It is only a small percentage that are dedicated enough to care, either because they really enjoy the game or because they want to support game preservation.

If you make it opt-in, companies won't care to add it and they will only lose a few percentage points (I would be surprised for anything higher than 1%) of people that will be put off by the lack of "EOL support" guarantee.