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In my experience (not actually me fortunately but people I know) it appears to be incompetence/laziness (probably due to using very junior staff to get a good recovery on the fee) or collusion.
A friend bought a flat as part ownership (a new complex so a lot of flats being sold). It was strongly suggested (by the vendor) that he use the solicitor recommended by the vendor (to the extent that he was concerned about getting the flat if he didn't). The vendor also gave £500 cashback for using their recommended solicitor.
The solicitor was terrible. Quite apart from getting the address of the property wrong, the contract summary they sent to him quite clearly wasn't based on his contract (it referenced clauses that weren't even in the contract) and a number of answers to his questions made no sense.
Most likely they were just incompetent/lazy and they were recommended by the vendor as they couldn't be bothered dealing with multiple solicitors but the whole process was very dubious looking at it from an outside viewpoint.
As before, the real question is why solicitors aren't explaining the impact of the agreements these people are wandering in to.