It is not an uncommon occurance for a student to wish to cancel their lesson because they didn't practice that week. Apparently, this seems to be a reasonable attitude. After all, if the student hasn't practiced, what is there to do in the lesson, beyond just repeat whatever was done last week? The teacher will just chastise the student for not practicing, and nothing will really get done, because the student and the teacher will just end up going over what the student should have done that week. Some teachers even advocate this attitude, seemingly intentionally guilt-tripping their students by saying "since you didn't practice this week, you and I are now going to practice together."
Actually, I don't think it's a bad idea for the student and the teacher to practice together. Much can be illuminated by watching the student practice, and the teacher may not have been aware of problems the student was facing. As far as why the student didn't practice that week, there could be many explanations. Perhaps the student didn't know what to practice, or didn't know how to practice it. Perhaps they tried practicing, and there were too many problems to continue. Perhaps they got frustrated.
Much light can be shed on these issues if the teacher engages in an honest inquiry of what problems the student is having. There is really no place for blame here. The fact is that the student didn't practice, and if the student wishes to make progress, the teacher must help the student figure out where the problems are and how to solve them. Not practicing is no more a character flaw than is being bad at rhythm or not being able to play scales fast.
I do additionally question the extent to which students are required to practice. Especially with beginning students, why is there an expectation that they practice at all? Obviously, if the student is spending time working on piano apart from the teacher, they will probably make faster progress, but there are several issues to consider here.
First of all, who is to say that the student is practicing correctly? I don't know why many teachers take it as a given that if they tell the student to "practice this piece" then productive work will be done on it. The student needs to be taught how to practice.
Second, even if the student is "taught" how to practice, who is to say that the student is practicing correctly? The instructions may have been misunderstood, or perhaps the student does not wish, for whatever reason, to practice the way the teacher instructed. Perhaps the teacher's expectations are too high, or simply not appropriate for that student.
Third, whose expectation is it that the student make fast progress? If it is the teacher's expectation, or the parent's, but not the student's, there could be problems. In the case of an adult student, it could be both the expectation of the teacher and the student, but it might be quite unreasonable on the part of the student to expect so much of themselves.
Fourth, there are many things that simply can't be practiced. The main reason I believe it is good to take piano lessons, and not say simply learn on own's own through YouTube videos, is because it is essential to have an experienced pianist watch what you are doing, and steer you in the right direction. This is not necessarily actual formal instruction, but rather a transmission of skill that can only happen in real time, face to face. It is an experience, one which through much repetition, eventually should rub off onto the student. I want to practice with my students. This is not a punishment; it is a fundamental reason why I am useful as a teacher.
I believe we need to adopt an attitude of curiosity here, rather than blame, and not focus so much on whether or not students are practicing, but rather on forming clear goals and always engaging in a process of motivated action. I don't believe that it helps anyone to blindly set expectations without reviewing the goals and desires of those involved.
So this brings us back to the initial question: "why should I have a lesson when I didn't practice?" The answer should now be clear. There are many reasons to have a lesson besides testing how your practicing went. As a teacher, I am interested in talking with my students about their experiences and frustrations while playing. It does not matter whether or not they practiced. With beginning students, it is necessary for me to review regularly how they are playing, and actually less important for them to practice. The more frequent the lessons, the better.
I am not advocating that students do not practice, or that teachers do not ask their students to practice. But this all has to be put into the proper perspective. There is much that can be done in piano lessons beyond reviewing practice. Students who are paying for a teacher should expect that the teacher is providing an experience that they could not get on their own.