Texters use deviant spellings - and they know they are deviant. But they are by no means the first to use such nonstandard forms as cos "because", wot "what", or gissa "give us a". Several of these are so much part of English literary tradition that they have been given entries in the Oxford English Dictionary. "Cos" is there from 1828 and "wot" from 1829. Many can be found in literary dialect representations, such as by Charles Dickens, Mark Twain, Walter Scott, DH Lawrence, or Alan Bleasdale ("Gissa job!").
Sending a message on a mobile phone is not the most natural of ways to communicate. The keypad isn't linguistically sensible. No one took letter-frequency considerations into account when designing it. For example, key 7 on my mobile contains four symbols, pqrs. It takes four key-presses to access the letter s, and yet s is one of the most frequently occurring letters in English. It is twice as easy to input q, which is one of the least frequently occurring letters. It should be the other way round. So any strategy that reduces the time and awkwardness of inputting graphic symbols is bound to be attractive.
Abbreviations were used as a natural, intuitive response to a technological problem. And they appeared in next to no time. Texters simply transferred (and then embellished) what they had encountered in other settings. We have all left notes in which we have replaced an and by an &, a three by a 3, and so on. Anglo-Saxon scribes used abbreviations of this kind.
But the need to save time and energy is by no means the whole story of texting.