Because it's significantly easier to just ask the phone to make a call, than it is to build VOIP calls into the app. As well, you get a lot of things for free:

Because it's significantly easier to just ask the phone to make a call, than it is to build VOIP calls into the app. As well, you get a lot of things for free: - user is familiar with the phone UI, and knows how to mute/use speaker phone. - put the call on hold if another incoming call comes in, - able to call someone and make it into a conference call, - more reliable on shitty internet.