It's only $69 to replace the 12's screen, so by the time you also rent the tools it's cheaper to have apple do it. https://support.apple.com/iphone/repair/service/battery-power
...just like it's cheaper to take your car to a professional mechanic when the gearbox needs changing, if it's only going to be a one off. That was true even before manufacturers started playing dirty tricks with proprietary engine management systems. If you're an enthusiast who enjoys fixing cars then maybe it's worth your while paying a ton of money to buy all the tools and dig an inspection pit in your garage.
I get the impression that these kits - particularly the tool rental - are a bit of lip service to the "right to repair" movement. A more interesting question is how much it costs to buy these tools - or lease them long-term - so serious hobbyists and small repair shops can offer a repair service.
If people really want the right to repair phones and tablets without needing more than a screwdriver then they'll probably have to put up with somewhat thicker and heavier phones - maybe not unreasonably so, but you're at the mercy of the herd who will head for the glued-together model that's 30% thinner, leaving the chunky repairable version as an expensive niche product. The consequences of ultra-miniaturisation often mean that you really need a custom jig or similar to efficiently & reliably work on the device. Even with batteries, making a user-removable battery means extra weight and space for the clips/connectors/hinges/whatever plus an extra layer of rigid protection for the battery to protect it when not inside the device. One reason glue is used is that it takes up less space than screws...
There's less excuse with Macs - certainly any Mac bigger than a Macbook Air - that simply don't need to be so thin, and Apple do seem to be rolling back from the absurdities of the 2016 MBPs there. Aside from the fact that so much of the functionality is built into the SoC and other components that need to be surface-mounted on the mainboard, the newer Macs seem to be pretty straightforward when it comes to replacing breakable bits like ports, displays, batteries, fans etc. - even SSDs on the Studio - once Apple starts selling the parts.