Because it is frequently used in literature.

Thanks to all for helping me!!! @Anony-Mousse: I have treated the ordinal data (1: strongly agree, ..., 5: strongly disagree) as intervall data and then performed the pca. Afterwards I have done a cluster analysis (k-means) on the raw data as well as on four factors. Both results were quite similar.

Thanks all for your answer. However, I'm still not quite sure What you mean with "treat Likert-type rating scale as interval"? Should I convert them by using e.g. optimal scaling or set the level of measurement for my variables as metric (instead of ordinal) in spss? Thanks!

Thank you, RubenGeert. By "treating such Likert scales as metric", you mean I can set the level of measurement for my variables as metric in spss? What kind of cluster analysis (hierachical,two-step) is the best, after having run a PCA?

Thanks a lot for your answer! I have another question: how to convert ordinal data to interval data in spss? Can I use "optimal scaling" or is there a better solution?