There are two reasons that I wouldn't expect a CDN to allow you to specify the origin server by IP address:
- Most websites are not accessible via IP address. Most hosting is on shared hosting where every IP address has many websites. Even if you have dedicated hosting with your own IP address, you would usually want to be able to serve several of your own domain names from your servers.
- Security certificates for IP addresses are are uncommon and expensive. (Free LetsEncrypt certificates are not available for IP addresses, for example.) Most users aren't going to be able to enable HTTPS to the origin server by IP address.
Because there are few users that would have a dedicated IP address just for one domain and be willing to pay for the the certificate for it, it would make sense to simplify and only allow host names and not IP address. It would be less headache for them not to have to support IP addresses and prevent problems of users trying to use an IP address that isn't likely to work.
If you do have an IP address just for your site, the workaround is to create a host name for it. For example you could create the subdomain
origin.example.com on your domain
origin subdomain would point to the IP address of your server.