That's because it's familiar and comfortable. Once you get hang of IPv6 you dislike the idea of fiddling with numbers as protocol takes care of most of things.
The protocol takes care of most things if default settings (e.g dynamic addressing, subnetting) suit you.
Predictable addresses are messy. Changing ISPs is a nightmare unless you run NAT. Firewalling being decoupled from NAT is a more complex experience. Memorising addresses becomes much harder. Old devices have varying support and common brand routers still have various IPv6 bugs.
I have IPv6 running at home and love it, but familiarity and comfort aren't the primary reasons it's less managable. Many existing configurations simply don't translate across very well.
This is quite a loaded question. You need some sort of IPv4 to access the IPv4 Internet. You need some sort of IPv4 if you have devices that don't support IPv6.
What you need in each case depends. If it's just to access the IPv4 Internet, you might get away with NAT64 and a single public IPv4 on the WAN interface.
> my static ip addresses are ipv4.
Ok? You can have static IPv6 addresses if you want.
That's because it's familiar and comfortable. Once you get hang of IPv6 you dislike the idea of fiddling with numbers as protocol takes care of most of things.