They can be, though aren't always, easier for two reasons. The first is that companies use the phone screen as a way to ensure a candidate reaches a basic threshold before spending the time and money to bring them onsite. So, in that vain it's more preponderance of the evidence that the candidate is good rather than beyond a reasonable doubt (which is the standard of the onsite).
The second is that the phone screen format isn't ideal for either the interviewer or interviewee and the questions themselves are sometimes a bit easier as a result.