This is probably because:

- IMs are already formatted to show the end of each message (chat bubble, etc), so a period communicates extra 'finality' beyond what is necessary;

- Sentence fragments are normal and expected, but seem weird with a period at the end;

- Using formality in an informal setting can create a sense of emotional distance;

- IM cues like these are likely established and spread by people who have mostly used IM in non-professional settings, with friends and significant others, and those norms are then brought with them when interpreting the IMs they receive in a professional setting. If you never communicated with your significant other via text message, you've probably never needed to express as many subtle emotional signals into your texts, and so just treat them like emails. But if you have, then you eventually pick up how powerful punctuation can be at communicating emotion in that medium.

Eg.

> I'll be home late tonight

>> Okay

VS:

> I'll be home late tonight

>> Okay.