This comes from the Sefer, Halacha Berurah from R’ Dovid Yosef.
Q: Is it permissible to pray the Amida while wearing shorts?
A: Only in areas where it is customary to wear shorts in the presence of prominent and distinguished people, such as in agricultural communities; even in such cases, however, one should not serve as Chazan while wearing shorts (Halacha Berura).
I cant put words in his mouth, but I am sure the Rav would agree that if it is not customary to wear a hat and jacket in the presence of prominent and distinguished people then one does not have to wear them by tefilla either.
Where R’ Chaim Kanievsky lives, it is customary to practically live in ones entire life in a hat and jacket while this is not the case in MANY places around the world. R’ Chaims psak is certainly appropriate for Bnei Brak, you can not foist it on the rest of the world and then say “what casting aside the words R’ Chaim Kanievsky?!!” In fact, I would bet that R’ Chaim himself did not intend for his psak to apply to the entire world.
Where I live, and daven, it is customary to greet distinguished people wearing a suit, yet I dont see anyone wearing a suit to davening, well, except the Rabbeim and a few kollel fellows who always wear ties.