Maybe in part because many therapists don't want to encourage setting strong boundaries but use whatever emerges as therapy material. And/or they know that most likely they would not stick with the same boundaries forever, so better not to get there unless the client requires it. I most often don't set clearly spoken boundaries with people but address it if something makes it relevant and necessary.

I think my last T really avoided addressing boundary issues. He did not do anything intrusive, but was quite unhelpful when I had strong ambivalence around emailing, for example. Mostly just said it was okay. But when I ended therapy with him, he would still engage in emailing for quite a long time. Nothing excessive, and all of his responses were quite professional and short, but still. Then one day just stopped responding altogether, without ever discussing it. I could never clearly figure out what this was all about but I decided not to care because I think I could speculate forever. I did try to get him tell me how he perceived me in general, just before I stopped seeing him, and he refused even that direct inquiry. He advertises himself as a very interactive T, and he indeed was, I liked that about him for the most part but sometimes his self-disclosures and excuses got a bit too much. The most I can speculate is that he basically enjoys the interactions and stops when that is no longer the case, maybe.