I agree there are times when it's more cost effective to just buy something rather than spend hours making it. But there are times when you need a tool and there is nothing other than a kit available regardless of price. I made the MLA-21 collet chuck for my Seneca Falls machine after looking for nearly a year for a commercially available chuck under $500.00. There weren't any available, especially with a 1 7/16-12 back plate. The only recourse was to pay huge dollars to have a commercial one made to order, or make one myself. If I recall correctly, at the time, the kit cost about $125.00 with shipping. It took about 2 weeks of spare time to complete it. It was a huge improvement over the 2C collet system the lathe originally came with. The 2C collets only go up to 9/16" and they are no longer available except by special order from Hardinge. The new chuck allows up to 1 1/8" standard collets, plus step collets that can handle up to 3".
On the other hand there was a time when I was looking for a die filer. I considered building the MLA-18 from the Metal Lathe Accessories website. However at the cost of $181.00 plus shipping I was able to find a used commercial unit in excellent condition, complete with a motor and stand for much less. I couldn't justify spending the money for the kit, and still have to buy a motor and make or buy a stand. I would consider other kits if the cost to build a particular tool was less expensive than buying it.