Why on Earth would someone break their neck from a crash? Have you ever heard of that happening?Safety. Same with the bike weight, and certain body positions on the bike - "super tuck", "superman", and bicycle geometry.
Imagine Ganna TTing on a futuristic Obree-style minimal carbon bike without top tube weighin 5kgs or something (I don't know what's the current top of the line standard for non-UCI legal bikes), and holding the handlebars with 50cm of spacers on top.
Maybe he can handle it, but someone will crash horribly doing that and break their neck.
I do think bike weight restrictions should be reviewed though. I think you can safely go under 6.8kg, probably significantly so, especially for smaller frames.
eta: although, if I got to be dictator of the UCI for a day, I would force everyone to ride steel bikes exclusively.