Because those of who grew up communicating a lot via the written word stopped feeling beholden to type using classic grammar rules like ending every sentence of every communication with a period no matter what.

The entire purpose of language is to express yourself, and people started noticing that their texts sounded friendlier if they sounded less abrupt, so they started typing that way.

You type according to traditional essay writing rules which is how older people learned to write, younger people learned to focus on producing natural sounding language and conversation.