Spiritualism seems to consist mainly of talking to dead people one knew when they were alive with little else behind or beyond it.
How could one make a religion out of this?

Spiritualism is an extremely limited aspect of our existence, not sufficient to guide us.

When these dead people were alive we probably did our utmost not to take their advice or listen to them, 'I am me!', and now that they are dead one wants to do what we avoided all along? LOL