Shortly: it is a limitation.
More info here.
Interesting.
Can’t some UDIM textures be larger than 14MB?
A HDRI environment definitely can be over 100MB and still be packed.
Also, we have .blend files at work which are larger than 1GB.
The last project I worked on was almost 2 gb.
But yeah, I have to agree with you, sometimes Blender acts itself pretty weird.
At issue is the type of file, not the size.
Not all external files can be packed
Some typically heavy external files, like videos from the Sequence Editor or Movie Clips, cannot be packed in a blend-file.
From the docs
I bet this is because videos usually are much larger in comparison to the same HDRis, so that’s why they did so.