Can you explain the reasoning behind this? Is it because unis frequently change their suggested reading lists?
It's because reading lists are partly a financial agreement between universities and bookshops.
Spending money in advance of your course on books you can loan for free from a library that may well be of very limited use is money down the drain.
When you start your course and speak to lecturers and students in later years you'll know which books are useful, whether they're in plentiful supply in the university library and what a good second hand price is.