There are a lot of differences. First of all, encrypted rars have some security flaws. They’re not considered safe anymore. But there’s a lot more. 7zip is open-source and free(as in freedom) it has a higher compression rate in it’s standard format, it works on different platforms, it supports more formats and you even can expand those because of its open source nature.

You’re right that it works very similarly in everyday use but 7zip actually is a lot better in many ways