Why Do Venomous Animals Live In Warm Climates?
Venom is the ‘lazy kill’. Not much energy requirement in producing toxins, huge energy requirements in active hunting.
Warm climates do not produce food with high energy content [lots of carbs and some calories, very little fat/protein], hot weather favors smaller animals.
Net result is that predators use venom rather than speed, strength or claws. There is no answer for the platypus. I think they are just showing off.