As per title really, why are seat belt bolts and sleeves always UNF? Why can't you use metric bolts?

You can use metric - can't remember the recognised equivalent just now, but as long as it's the right size & grade then it's OK.

Its down to US Federal Law, the requirement is 7/16 UNF or 1/2 UNC.

I believe it has been amended to say or metric equivalent now, but guess manufacturers just stuck with existing tooling.

The clio has m10.

It's so you can use Mini wheel nuts when you lose the proper ones......