The requirement is not exactly "iOS or Android", the requirement is "SIM card with valid phone number". Otherwise, you could use it on iPads and Android tablets, which you normally can't.

WhatsApp accounts are directly tied to a single phone number, both for user discovery (that way, you can simply message everybody in your contacts who has the app - just the way user expect it to work) and for spam prevention.

Creating a smartphone messaging app without this feature would be orders of magnitude more difficult, you simply can't get normie users to go around "hey, what's your WhatsApp user name?"