It's the case for many "floating ip" style systems, like EC2 and Openstack too. NAT-based floating IP style systems have some great benefits, in particular tha

It's the case for many "floating ip" style systems, like EC2 and Openstack too. NAT-based floating IP style systems have some great benefits, in particular that the cloud server itself doesn't need to know anything about them. So you can move Cloud IPs between them without changing any configs, or doing anything particularly special on the servers themselves. Adding multiple IPs to a cloud server adds quite a bit of complexity - makes it harder to just move the floating IPs between servers (because you have to maintain a mapping to each IP, which is different on each server) and it makes configuring the web server more complex too (because the IPs will be different on each server). In the majority of use cases I've seen, you don't actually need multiple IP addresses on the same cloud server, so it's overkill to support them when port translation can solve the most common problem so neatly.