I like kits because I don't have the tools or work space to roll my own.
Many people don't have the specialty tools needed to build there own kit parts especially fiberglass ones, light tools are extremely limited in their capabilities unless you have lots of time, kits save time and as others have said you can kit bash into other designs.
With all due respect, you don't need a lot of specialty tools to fabricate from scratch. No question they can make life easier, but you can do it with a handsaw, a couple of files and a drill - all stuff I'd think a rocketeer would have anyway, and at the absolute most it's the cost of a couple of G through I motors (unless you choose to spend more). When I make a tube I use paint brushes that you can get from any hardware store, and the 1" tubing I use for a stand I also use for prepping rockets in the field - in total I think it cost me $10 (if that). The allthread that runs through the mandrel was the cheapest thing at Home Depot longer than 48". The workbench I have is Stanley plastic collapsable workbench, and what is special about it was that it has clamps and was cheap. Rumour has it that Jim Jarvis rolled his 4CarbYen tubes in his laundry room. Specialty tools and lots of space definitely make things easier, no question, but you don't
NEED them if you want to scratch build.
Having said the above, I'm just going to throw it out there that you will not save money fully scratch building. Yes, you can make rockets out of Pringles cans (etc) but if you love rocketry as a hobby I'd bet you'll run through those options fairly quickly. If you're interested in efficiency (height/speed for a given weight or given motor) I know you'll run through your options quickly. I've sourced the fiberglass cloth, laminating epoxy, peel ply and shrink wrap (etc), done it on the cheap (without an oven/vac bag, etc) and the all in cost for a scratch built fiberglass tube is still more than the retail cost. Carbon tubes are a bit cheaper to a lot cheaper depending on the what and the how, assuming your build abilities result in a comparable tube. (IMHO saving money by making an inferior product is short term thinking.)
Why do a kit? They're cheaper than a full scratch build, especially when kits are on sale, and it takes a lot less time for a build. If you like to build but you don't want to worry about design (and worry about CP/CG and do the rocket science bit) they're a great way to fly some birds and enjoy the hobby. They're also a great way to source parts if you do want to scratch build but don't want to fully fabricate from scratch.
Why scratch build if it's cheaper to buy the stuff in a kit? Speaking personally, I have longer term aspirations of an L3, full scratch, full carbon stratospheric bird. Yes, I could buy the stuff, but I see value in being able to look at it break 50,000 feet and say 'I designed and built that from absolute scratch." I'm practicing on FG because it's cheaper and all the lessons learned will apply to carbon, and what I'm learning will allow me to build the custom parts I might need. And yes, I fabricated all the external and internal components for both my L1 and my L2 birds (save for propellant and electronics), and I'm proud of that. Last, I like the design aspect of this hobby - CP, CG, stability, fin design, heat tolerances, the whole lot of it - and you don't get that with a kit.
This. If I had a lathe I wouldn't make that statement, but... I don't have a lathe... See point above about being able to do it without specialty tools - you can do it, it just takes a bunch more time and effort to get it right.