Because all devices do not have the same power needs. Some are AC, while some are DC. Some require 3 V, while some require 30 V. Some require 5 A, while some only require 5 mA. Some require a regulated supply, while some don’t care. And if you haven’t noticed, there are exactly 3 million different kind of connectors used w/ “wall wart” power supplies.
At the very least, making such a “universal / multi-channel” power supply would require that:
- each channel have a voltage adjustment
- each channel AC/DC selectable
- each channel be able to deliver a substantial amount of current
- each channel be regulated
You would also need a dizzying array of adapter cables.
Very impractical, to say the least…