This is done mainly to strip off meta/EXIF/GPS Data, and mitigate other format violations often used to cause "trouble". Resampling/transcoding also tends to corrupt steganography utilities, thrash user hardware thumbprints, and standardize the web experience by excluding unsafe formats in favor of well-tested efficient traditional codecs.

Specifics preclude the multifaceted nature of the policy.

https://www.youtube.com/watch?v=cJMwBwFj5nQ

Happy computing =)