An ASN with a /32 allocation (the smallest for ISPs) is four billion /64s. It takes dozens of yottabytes of traffic to exhaustively scan one single /64. The entire v4 space takes 0.00000001 yottabytes, or about 110 GB/port in more understandable units.
There's a ton of things you can do to cut down on the scan space for v6, but it's still far huger than v4 can be.
