The difference in data rate and SNR required for voice and text messaging is enormous – if you can afford for your message to take a couple of seconds to go out, you can be fine without an antenna.
I currently own a couple and have owned a bunch of other devices in this category. Have you ever looked at a tear down of one of those devices? Now compare that to a tear down of an iphone? Even supposing Apple can design a much smaller antenna, where is it going to fit? What’s the battery life going to be when it has to send messages periodically (until confirmed) at MUCH higher output power?