Amtrak does plenty of interlining tickets. They're called Thruway buses.
There are a few train services to which you can connect (such as NJ Transit to Atlantic City).
They do some. Whether it is plenty or not is in the eyes of the beholder.
I guess I am completely spoiled by experiences in Europe.
In UK for example I can get a ticket from any London suburban station to say Edinburgh where the trip involves multiple TOCs. What I would like to see is that I can get a ticket from the station close to my home, which is an NJT station to anywhere where Amtrak goes or any other transit agency that connects with Amtrak goes. How difficult can that be in this day and age?