Why wouldn't they? The sums work differently for everyone.
Fox's would probably want their chargeable rate of say £70 an hour for their competent welder to do it (whilst paying him £40 an hour), so for them it made no sense. But if the welder himself fancied a bit of extra work on his own account at the weekend, he could do the whole job for no more than the cost of the steel, the welding wire and the electricity, and get himself an almost 'free' boat. But as john.k points out, the lost scrap value also needs working into the sums.