Becasue the term "Asian" means anyone from Asia - hundreds of different cultures. Why do Brits narrow it down to just three? Especially when actually referring to Muslims? Most Asians are Hindus or Buddhists - NOT Muslims, so why use the word "Asian" when you are only referring to Pakistanis or Indians?
That's like referring to Brits, Greeks, Russians, Swedes, Irish, Germans, Italians, Hungarians and Serbians all as "Europeans".
While it is true that a Swede is a European - it is NOT true that a European is just a Swede.
Same thing with "Asian" and "Muslim".
"Becasue the term "Asian" means anyone from Asia - hundreds of different cultures. Why do Brits narrow it down to just three?"
Because those bits of Asia are the bits we used to own and from where there was immigration into the UK in the postwar period. Hence most people's experience of someone from Asia would have been from one of those countries and hence the usage.
"That's like referring to Brits, Greeks, Russians, Swedes, Irish, Germans, Italians, Hungarians and Serbians all as "Europeans"."
Yes it is. Which people from the US do regularly! Why do American police sometimes describe someone as 'black' when they are actually of Jamaican descent?