There may be social reasons to do it, but then, of course, this should involve some sort of subsidy to the carrier to make up the loss.Why is It children are discounted at all? They take up the same amount of seat space as an adult.
In this case, without some sort of family discount, Mom and Dad might decide to drive the kids up to New York the next time, thus contributing the the already bad gridlock on the New Jersey Turnpike and affecting various clean air targets. It's already far cheaper for 4 or 5 people to travel by car than any other sort of public transportation, this may conflict with Clean Air Act implementation plans that propose to reduce VMT to improve air quality.
As for seniors and, especially, disabled, many are not able to drive, which means that making it more expensive for them may unduly restrict their mobility. (though, let's be real, the discounts are not that big.) There's a reason why transit agencies around the world offer senior and disability discounts. Again, there may be a social reason to offer incentives better enable mobility for seniors and disabled, but perhaps it's not up the the National Railroad Passenger Corporation to be funding those incentives.
By the way, there is no senior discount for the Acela Express, at least on weekdays.