Tim,
You answered your own question: "zero to five percent."
I don't think it's the architecture at all, though the routing and topo of some courses are indeed unwalkable.
Cartball is simply a popular game in America because a) it enhances the financial viability of public and private courses, b) people can afford to ride, c) people are lazy, d) they didn't grow up walking and carrying like I did, and e) let's be honest, driving a golf cart is fun, particularly for NASCAR fanatics.
I can't see laying the blame at the feet of the modern architects. I suspect few people walk Ross' Hill Course at French Lick, Flynn's Lower Cascades Course at The Homestead or Langford's Gatlinburg Country Club, just to name a few older courses.
Regards,
Mike