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It's not inefficient, it's extraordinarily efficient.

There are efficiencies and then there are relative efficiencies. You could design a combustion engine that used its intake air very efficiently, but as our supply of atmospheric oxygen is effectively limitless, it would be completely irrelevant. No one is ever going to care how efficient their car is at managing its oxygen use.

Posting images of text is not cheap in terms of computer resources, but it's extraordinarily efficient in terms of human time. A quick screen shot, a box to select, and you're done. Sure, you could just as quickly copy the text of a tweet or other message, but you would also be cutting out important details. If you want to include the source's username, the site it came from, the profile picture, etc., each of these has to be gathered separately. Then they have to be pasted into a text entry box, as well as any images. Oh, and it won't actually end up being formatted like it was on the original site.

People post text as images for the exact same reason we invented computers in the first place - human convenience and time savings. If you want to post text that preserves its original context and source, it is almost always easier to just post a screen shot than it is to manually copy all that information over piecemeal. Yes, posting the text directly is more resource efficient, but that's pretty irrelevant in 2026. You do have points on the translation though. And of course it does make it harder for screen readers.

But there's your answer. Convenience.