Ask Lemmy

because the internet is just six social media platforms and each is full of pictures of posts from the other five

I can guess a few things. For one, its slightly less effort than selecting the chosen text and copying, though not much less so I don't think that's the whole reason. But also, it does technically give more information that otherwise would have to be typed out, like its often possible to tell what platform the text comes from, the username and profile icon of the original poster, etc. And if there's a chain of several people responding, you can capture the whole conversation without worrying about formatting nested quotes and such. Fonts can also convey mood, which you'd lose if you just copy the text.

Finally, I guess it could feel more clear as to the source, like if you copy paste text with some quote formatting, there's more room for someone who doesn't understand the formatting to think you were saying that thing, and it's immediately obvious that you could edit or make up the quote if you wanted. A picture immediately says "this is something this other person somewhere else said" in a way that's harder to misinterpret and looks harder to fake (even if in reality making a faked picture of text is easy)

I'd guess it's because people are either lazy and not wanting to type the text out, or people don't like / know about modern tools to extract the text for posting.

Also, meme culture doesn't have the same ring to it when the text is extracted from the picture.

Platforms want you to know which platform and which user the text you’re copy-pasting comes from, so they make it much easier to screenshot the text than copy-paste it. Users encounter this often enough that they don’t even bother on platforms that behave better.