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"Why don't reputable brands sell Frankenstein cables?"
The device will tell the charger how much power it wants. The charger will tell the device how much power it can provide. What do you think will happen when 3 devices all start talking over one another? If you connect a laptop and a camera, then either the laptop is going to take 3 days to charge or the camera in in for a fun ride.
You can't really do it in a way that's compliant with USB specifications. USB is a purely point-to-point connection, a cable like this that offers even a minute chance of having three or more devices connected to the same conductor is just asking for all kinds of trouble. Even with just two devices, there have been issues with non-compliant USB-C cables causing damage; no reputable brand is going to risk making something even more complicated. If you need to deal with different connectors, you're just going to have to carry multiple cables; such is life until we finally get everything on USB-C.
Hold on a few more years and everything should be on usb-c.
Some have in the past and they tend to fail faster as they have more moving aka working parts, older USB cables specially mini USB tend to fail after while because of how they were designed with this little teeth. Even older lighting cables had this issue. Now with apple being forced to use USB C cables for universal charging. Things will be a lot easier