Most recipes are useless internationally anyway, the ingredients are often different locally or hard to source. If you're lucky and you have a kitchen in two different regions or, like me, you live in a non-English speaking country who still wants to cook dishes from home then it's going to affect you. That's a vanishingly small number of people.
Otherwise, most people will know how much an egg weighs, especially if they bake regularly.
Yes, this is a point that is usually neglected. People search for recipes online and assume they will all work. The flour is very different across the world. You can easily get Indian flours in the UK, but it's quite hard to find French and impossible to find American (it is considered not fit for human consumption due to the bleaching process).
Bleach is not generally considered OK to eat. The flour is naturally whitened by ageing and there are naturally softer flours so no need to chemically alter them.
No different than US volume measure where you actually need to have the "measuring cups". Weight is still easier, as you can refer to the reference range of 'large' and reproduce it on a scale.
Under a single region-bound regulatory body, making it useless internationally.