Because it is a LOT more complicated than that.

Not all rheumatological diseases are due to the immune system’s memory. As a case in point, Ankylosing Spondylitis is theorized as being caused by a mis-folded HLA-B27 protein response. The mis-folded protein response is caused by cellular stress, at least that is the theory. The lead singer of Imagine Dragon Dan Reynolds suffers from this disease. So there are people out there suffering from it, it’s not just some disease out in left field no one has heard of before.

Are there diseases that could be treated by clearing the immune system’s memory? Possibly, but there would also be consequences for that as well. Mainly, because the actual method by which the memory works is not completely understood.

Disclaimer: My wife is a Rheumatologist that does both basic research and clinical work. What I wrote above is based on what I have gleaned from her over the years. Any mistakes or misconceptions are strictly mine. I’m just an old IT guy and have never studied medicine.