The key-phrase is "in Islam"...
Trouble for you is that not all Muslims are "ideal followers" of Islam. Infact most are not.
Thats why, It wont change the fact that a Kashmiri will never look like a Bengali and their languages will always be different and muslims will feel closer to their ethnic equivalents (ie other similar ethnic muslims) than a distant and different muslim.
Why do you think that Arabs have their Arab identity distinct from say Bangladeshi identity? Both follow Islam yet they cann't be any more different.
You can believe whatever you want, it won't change your material reality. May be spiritual reality can change but certainly not material one.