Don't know of any lawyer who would write their own will. The old saying "The lawyer who acts for himself has a fool for a client" still resonates! If they did, they'd have it checked by another lawyer, out of abundant caution (or 'ex abundante cautela' as they might put it). They'd not be quite so fondly remembered if they missed something or made some slip to the detriment of their estate.
There are safeguards in using a solicitor. He's insured, for one thing. The will is less likely to be challenged by disgruntled relatives on grounds of duress, fraud, or undue influence, since the solicitor should satisfy himself that those are absent, which deters the intending litigant. And, of course, mistakes that the amateur can so easily make are avoided.