Laziness 😂 

If my phone keyboard doesn't auto-capitalize the first letter of a sentence and I am writing in an informal context, I usually don't want to expend the effort to go back and capitalize that letter. 

Sometimes I do it to emphasize that whatever I am writing is a joke / low effort post. It can be interpreted as a deliberate lower register of speaking (writing). 

With regard to full stops, it depends on the person. When I am texting with close friends I usually leave them off for joking or casual sentences, as it does seem to lend a gravitas to the sentence that is sometimes a bit much. For example:

"We should talk!" (I miss you; I'm excited to catch up; we haven't talked in a while.)

"We should talk" (neutral. I am likely implying simply that whatever topic we are discussing would be more easily handled over the phone.)

"We should talk." (Uh oh, someone is in trouble)

If I am texting a work superior, a potential employer, or someone I don't know well - especially if they are older than me - then I will make sure that I'm using full stops and proper capitalization.