The main reason behind the quality loss is the compression algorithm used by the different social media platforms.

Recently a famous tech YouTuber Marques Brownlee did a video on "The Blind Smartphone Camera Test 2020" in which he discussed this issue.

He said: "Both Instagram and Twitter compress your photos and show in a very different way"

On his video the discussion related to image compression starts from 13:40 minute. You can check his video here.