The answer to 'why is my bathtub foul smell' is that a smelly bathtub is often caused by a buildup of hair, soap scum, and bacteria in the drain. This can create an unpleasant odor that can permeate the bathroom. Some common symptoms include a strong, musty smell coming from the tub drain, as well as slow draining water.
There are a few potential solutions you can try before calling a professional plumber. First, try using a drain cleaner or plunger to clear any blockages in the drain. You can also try using a wire coat hanger or other tool to remove any visible debris. If the smell persists, you may need to disassemble the drain and thoroughly clean it. This can be a more involved process, so it's best to call a plumber if you're not comfortable doing it yourself.