Because that's what the owner, my FI and the checklist supplied with the aircraft told me to do. I imagine a lot of aircraft owners do the same without questioning why.
As a relatively low hour PPL holder I don't ignore that kind of advice because I don't know any better. That doesn't mean it's the right way to do things, so I'm always open to other opinions.
If it were my aircraft I would be keen to minimise engine wear and tear and I think I could manage to change prop pitch as well as flaps, gear etc when going-around, so it's certainly not something I would not consider.