Why can't a forward play more like a back

Put Kwagga at 13 is the latest cry.....I remember when many pundits wanted Spies at centre. What I don't remember is one forward who has successfully converted into a test quality back. But I do remember one back, Dave pick for Bok 12 Craig Burden, who became a fairly decent forward. So .....why can't a forward play more like a back: One back in a million may loaf a bit. Now and then there's one with slight defects; One, perhaps, whose qualifications you doubt a bit. But by and large we dominate the texts. Why can't a forward take after a back? Cause backs are so sexy, good-looking and kind. A better competitor you never will find. With apologies to Professor Higgins. Those slow, smelly bastards can never be more like a back. It's genetic.

Kolbe at scrumhalf is a personal pet peeve.
Examples like Dayimani where they are athletically gifted but lack the grunt and nuances of forward play may as well try the backline.