Solicitors do this because they stand a very good chance of making big bucks ........
I'm not convinced that that is a major motivating reason. There are other reasons. For the solicitor - particularly newly established, it can be a good way of meeting new clients and anyone who owns property or has money, has a spouse and/or dependents should make a will to avoid the occasionally very nasty consequences of dying intestate. If anything solicitors probably do better out of clients who don't make wills or make home made wills.
As a practising solicitor and like the bulk of my profession, I am happy to make simple wills for clients during the course of other transactions at no charge. For stand alone wills, there will be a relatively modest fee. I have stopped doing the Credit Union type blanket wills for free as (a) I personally found it quite time consuming, (b) I don't need the potential business that it might generate and (c) a lot of people wasted my time, delighted with themselves at being able to chop and change instructions and appointments and all for no cost. A lot of people behave quite badly when they are getting a professional service for nothing.
mf