Should edited, multi-author undergraduate textbooks have many in-text citations?
My view is that in-text citations to relevant, pedagogical references are very useful.  Introduction to Electrodynamics by David J. Griffiths is an excellent example.


  Are citations necessary in this case as for any other academic work? 


Yes.  They are still an ethical obligation, but they might not be in-text.


  Or do citations interrupt the smooth reading of the text for undergraduates?


Obviously yes, but in my opinion that is unimportant.


  Or are they less necessary (or less numerous) because the authors are experts?


The identity of the author is irrelevant to the ethical obligation to cite prior work.