It’s because it’s okay to post mundane things on Twitter/X. It’s because tweets are short and are very fleeting.
An Instagram post takes up my whole screen and a picture is expected. Each post is given so much real estate and it makes you want to dress it up.
In the end, those different amounts of “friction” lends to posting different kinds of content.
It’s a vibe of a high end dinner establishment vs. a quick pickup place. They have their own lanes.