It is to do with being informal. Keeping an instant message conversation without a period makes you appear more open to conversation. Using proper punctuation makes you appear quite unapproachable and too serious all the time.
I use punctuation marks and capitals in the correct places religiously in texts, emails and WhatsApps, either as a result of having English grammar hammered into me at school, or because I’m a dyed in the wool pedant.
All my friends and relatives keep replying or sending stuff to me, so maybe I am approachable, but to be fair, quite a few of my in-laws are as dumb as all get out, or possibly certified dyslexics, e.g. consistently typing, should have and could have rather than should have or could have.
A classic from my wife’s sister-in-law, (not my sister), “Jean, sweetheart, can I lend your leaf suction thingy?”
“Sure Lesley, who are you going to lend it to?”
She came back with, “You ******, (rhymes with banker), can you borrow me your leaf suction thing, yes or no?”