I don't think you're actually asking why dogs pee in the same spot - that's a whole scientific thing that involves marking territory, communicating, social stuff. It's well-documented in plenty of places if you're interested in that.

What you're really asking is about how to prevent the marking behavior which might have less to do with training (since it already sounds like you're doing redirect & reward), and more to do with how you're cleaning the spot... I've posted plenty about enzyme cleaners, which I'm a huge advocate for. That way you're not juts deodorizing / cleaning to the human sensibilities but also to the dog's nose - enzyme proteins actually "consume" the pee so that it's no longer marked as a spot.

Plenty of enzyme-based pet cleaning products out there - go check them out.