Why all old female playback voices look similar?

Actually, two or three female singers used to be a little nasal and they sounded very similar. I always fail to recognize their voice but speaking about Lata and Asha, they are not at all similar.  

I don't see any such similarities in old  Playback singers. Mukesh, Rafi,Mannadey or Kishore have different and unique voices of their own. Even in South Suseela and Janaki have different voices of their own .For example Mannadey's Pucho na kaise... is  very different from Rafi's Chahe Koi mujhe Jungilee kahe...Yes, I agree Mukesh's voices do not match with some actors on the Screen. In this respect Rafi's voice suits almost all the actors on the Screen.

 I hold a totally different  opinion about this. I feel that all have their unique voice texture and singing style. Kishore, Rafi, Mukesh, Suresh Wadekar, Manhar Udaas, Manna Dey....They all had their uniqueness. So was with the leading ladies - Suraiya, Geeta Dutt, Lata and Asha. 

However, Later on two schools got established---One following Kishore's style and other one following Rafi. And so later singers like Kumar Sanu, Udit Narayan and Sonu Nigam followed one of these schools. Females always got inspired by Lata and Asha and tried to imitate their singing styles. However, the ones like Sunidhi Chouhan and Rekha Bhardwaj have their unique texture and style.

Almost all the yesteryear singers were unique in their own way. All the male singers had different singing style and so did the female singers. The prominent female singers were Suraiya, Shsmshad Begum, Lata Mangeshkar, Asha Bhosle and not to forget Geeta Dutt and no two voices were similar except may be Suman Kalyanpur who sounded a lot like Lata..all of them could sing all kinds  of songs. In fact the club songs were normally given to either Geeta Dutt or Asha Bhosle but Lata too sang quitea few of them..

I feel that singers in general are unique in their own ways and they have their own expertise. We mostly tend to note them as the voice of particular actors and many a times they do get stereotyped. Singers do have a lot of range and are very talented.