Using correct grammar requires training and effort.
Proper grammar is important for clarity, especially when writing for a one-way medium that does not allow the reader to ask for clarification.
For example, using good grammar is important for books, magazines, and newspaper articles, because the reader does not have the opportunity to ask the writer what he or she really meant.
Sentences in published writing must be unambiguous, and that is one of the reasons grammar rules were formulated.
However, in casual conversation, if we don't understand what someone means, we can ask for clarification. The speaker can easily repeat or rephrase what they previously said.
Text messaging is more like a having a casual conversation than it is like writing a book. If a text message is unclear, the recipient can ask for clarification.
Therefore, people tend not to fuss too much about grammar in text messages. Maintaining good grammar takes time and energy. Text messaging prioritizes immediacy over clarity.
For many people, using correct grammar is simply not worth the added effort.
Also, few people are grammarians. But many people want to text message. If only grammarians were allowed to text, a lot of wireless companies would go out of business.
I'm guessing that the person who wrote that sloppy text message uses better grammar in more formal contexts, like school papers.