You’re using concepts that aren’t usually used together. Spiritualists, of which mediums are a type of, generally don’t found churches. Prophets found churches. There’s a pretty big difference between the two. Spiritualism is generally something you do on your own or with a small group of people. A small group of spiritualists practicing together is generally called a cult rather than a church. Cult beliefs don’t have to be backwards compatible like religions do, so they change far more frequently.
Prophecy tries to tailor its message to the greatest number of people. Prophets have to be wise and prescient in order to gain people’s respect. In the old days, you also had to have political support. Christianity had a wonderful prophet, but it too would not have gotten where it is today without Constantine’s support.
In the old days, trying to start your own religion was called apostasy and was very often a capital offense. There were specialized executive bodies called inquisitions that would travel around looking for them. So the mystically-bent would instead gather small groups and do their thing out in the woods late at night where nobody would catch them. When they could count on the townsfolk to look the other way, these occasionally got rather large and today we call them mystery traditions.
Reincarnation is an eastern idea that didn’t gain much purchase in the West until the nineteenth century. I can only theorize as to the reasons, but here is my theory: The concept of afterlives was simply just more appealing to Westerners due to the greater level of civilization that existed in China and India 2–3000 years ago. If you have a safe, comfortable civilized life, you’re more likely to want to continue living there rather than conjure up a perfect afterlife to want to live in.
That said, the idea eventually did cross-pollinate among the intelligentsia of the West and the esotericists that popped up in elite society, once religious authority lost its grip on public morality.