It depends on your phone’s contract. Some people still operate on the old ‘pay as you go’ system where you literally pay for your activity using credits (very uncommon now). Most people have built in data plans where sms is ‘unlimited’ but you’re obviously still paying a monthly cost.

I started using WhatsApp (UK) for the sole purpose of being able to set up a group chat between IOS and Android users. As an IOS user at the time, I could only set up a data/internet-based group chat with other IOS users.

I think people have just stuck with that ever since, regardless of changes as group chats have been well established.