In many European countries, PhD salaries are incredibly low if compared with industry salaries. In my field, computer science, a young programmer can gain in one in one year what a PhD student gains during all the PhD. Which are the practical reasons of this?
Money: university have not enough money to pay higher salaries to PhD students. I don't think this is the cause of the low PhD salaries, as given a budget for a project, the PhD students are hired in function of the budget.
PhD is not considered a job. This, in my opinion, is wrong. PhD actually is a full time job, that requires skills that in many cases (for instance in computer science) will be highly paid in the industry. Projects faced during the PhD are often more complex than homologous industrial projects. The real fact is that a PhD student often (but not always) doesn't produce revenues; but this is true also for professors, for people who take care of the cleaning of the university, and for all the staff of a university, that receive a higher salary then PhDs.
PhD students are the weakest group of the academia. Nobody, when salaries are decided, says "PhDs should gain more!", because, when salaries are decided, actually they don't have any voice.