On the other hand, why pay for stuff you don't need?
Because the carrier requires you to. For a long time, carriers would not activate low-minute pay-as-you-go plans with no data on smartphones. Only dumbphones were eligible for plans with no data. Instead, carriers required customers to have high-minute or unmetered plans including data, even if the subscriber already has a landline at home and plans to be near Wi-Fi whenever doing anything requiring an Internet connection. For example, Sprint's Virgin Mobile wouldn't activate a payLo plan on an Android phone, and AT&T has been known to cram a data plan onto a subscriber's bill unless the subscriber performed some obscure trickery involving buying a SIM online and activating it through the Internet before putting it in the phone.
Or because manufacturers have required you to. There hasn't been a serious Android-powered challenger to the iPod touch, a Wi-Fi-only tablet in the 4 to 5 inch range. The Archos 43 never caught on because of its resistive touch screen (though it worked well with a stylus) and lack of Google Play Store. Samsung tried with the Galaxy Player, but it didn't catch on either, possibly in part because it was stuck on Android 2 and because I never saw them in stores. So people who never plan to connect to a cellular network end up paying for a cellular radio that they don't use.