Tablet computers are designed to perform different functions beyond simple telephony. They are computers, not just telephones, and the two functions fall into distinct categories of technology and usage.
To be perfectly accurate, modern "smartphones" are more tablet computers with additional telephone function, than they are telephones with computer functions. People no longer merely carry phones, they carry personal computers with the added capabilities of phones.
Yet despite this, there are still countless phones the world over which completely lack tablet functions - and there are likewise tablets that lack a telephone function. They are separate technologies, even if the recent trend has been to combine the two into a single device.
In contrast, PLBs, EPIRBs, and Satphones all perform the exact same basic functionality in the exact same way, communicating through radio signals sent directly to satellites. The only difference is how you choose to house the tech, with some models of course being compact and fragile, and others being more sturdy but bulkier.