There are sort of two questions here:

> why does EXIF transform metadata exist?

Because there are times when it is useful to be able to quickly and losslessly rotate a photo. For example an image viewer might provide quick options to rotate a photo (in case it did end up rotated the wrong way originally).

Probably this should just have been part of the actual image format rather than metadata, but that isn't how it played out.

> why doesn't the camera itself perform a simple rotation or mirror

There could be various reasons. The simplest is that adjusting the rotation naturally happens later in the pipeline than lossy encoding. You don't want to re-encode for quality and performance reasons so just slap on the metadata. The other could be that it is less expensive to avoid rotating. In many cameras the encoding is handled by specialized hardware, adding a way to rotate the raw pixels is likely more expensive than always having the source pixel map to the same input pixel in the encoder. There is no hard reason it couldn't be done, but in many situations it is cheaper to do it this way.