>> it doesn't resolve a domain name
1) Because it's not required.
2) IP is routable, not host.
3) Not everyone has control to their reverse DNS (PTR) because their ISPs don't delegate that.
4) Some ISPs for whatever reasons don't always set a PTR record for their IPs. You can say they have a broken PTR.
When you have a broken PTR, you can't run DNS (authoritative) server and shouldn't run SMTP on that IP.
>> do more people visit using IP only
Read (2). As long as that IP is routable and can reach its destination.