Because it doesn't really boost my productivity. I try to keep the number of windows opened to the minimum: Having more than 4 windows opened is counterproductive (at least for me) since it implies 4 different context I must switch between. I can handle 4 different windows opened easily without any tiling wms.

The windows I have opened are: - the terminal (at most 4 tabs as well... but here my terminal allows me to easily switch to any numbered tab I want) - the IDE (sometimes). In most cases, my IDE can be run on my terminal (e.g., Vim) - a web browser (again, I try to keep the number of tabs to a minimum... but here it's harder to do so) - anything else (usually nothing)

Another reason to not use a tiling wms: yet another dependency I don't have to take care (as in: configuring and updating it).