Because they're a historical legal document signed by a monarch, I think - it feels both a bit archaic and a bit special. Like, at least back then, and to some extent even nowadays, having a commission means you're an officer and that's a Thing, and I imagine they have some fancy crest and maybe a bit of calligraphy and a Seal of Approval or something like that. And a monarch/royal person still feels special to me on some level, despite my varying opinions about the actual ones floating around these days.

I hoped my degree certificate would be like that, and in the end it was printed on cheap paper and didn't feel as special.

I know there's nothing to stop me from buying thick paper from a craft shop and doing calligraphy and heraldry and sealing wax myself, but it has not yet happened. Some day I will make really fancy adulting certificates for my friends.