One reason I can see is it simplifies security. IPv6 being routed, it means you don't have the inherent firewalling provided by NAT for inbound connections. IPv6 requires you to manually configure it in some cases. It's easy to accidentally leave a device fully exposed to the Internet. While a misconfigured NAT would immediately be visible with loss of Internet access.
Depends on the vendor - but this brings up a really good point, in that there is a lack of consistency across the ISP's and the OEM's on how to configure devices - not pointing fingers, it's just that IPv6 has more options than a Porsche 911...
On the WAN side - it is SLAAC, DHCP, DHCP-PD, 6in4, 6to4, 6RD, IPv6 Relay and other options - and that's just to configure the WAN - on the LAN, it's just as bad - do we do stateless, stateless-dhcp, or stateful, just for IPv6, and that's on top of IPv4 settings for the LAN.
Then you have the 5G Fixed Wireless carriers like T-Mobile US - they use 464XLAT, and they don't assign PD on the WAN, which makes things a bit of a challenge for obvious reasons when one want to use something other than their gateway.
I can see why folks would just say "screw it" and turn it off...
At least with firewalls - OpenWRT does try to do the right thing with firewall support, as both IPv6 and IPv4 are treated equally within the rulesets via nftables and firewall4 - and flexible enough that one can set up IP version specific rules if needed. But this is the exception, and not the norm.