One of the contributing factors to this problem is the existence of so-called "claims farmers". These are (legitimate) businesses whose function is to introduce third party insurance claims to firms of solicitors/accident management companies, in exchange for either a fixed fee per claim, or a percentage of any damages awarded. In the same way, the solicitor/accident management company handling the claim will seek their own costs from the third party. The claims farmers play the volume game, i.e. the more claims they sell, the more fees they get, so there can be a temptation to sell legally-fragile claims or even outright fraudulent ones. The claims farming business model is often run on a telesales commission-led basis, so individual telesales agents can be tempted to do anything to get a sale, even by 'inventing' accidents. This is what gives rise to the telephone calls offering to make claims for an accident you haven't had. The situation can be compounded if the solicitor or associated party (medical 'expert', car hire company, forensic mechanic, etc.) is less than scrupulous and is willing to submit a report in favour of a dodgy claim so that the third party insurer will cough up their (often top-end) costs.
So, bear in mind that the lowlife scum inventing personal injury cases are not always individuals but can be so-called professional parties as well. Always best to use a trusted solicitor.