The reason is USB Power Delivery standard. You cannot make a product that is against a standard and the standard does not allow it.
Also, as per the name, it is a standard for delivering power, it is not intended to suit directly for powering random equipment with any arbitrary voltage or current levels - you ask for some power level, it delivers it, and you can do anything you like with the power and convert it up or down to any voltages you want inside your device.
So no device will request e.g. 12V/5A and thus no device needs to provide 12V/5A, there is no standard that allows it.
The standard started at some version and at some point was up to 60W using up to max 3A and up to max 20V and requires the device to consume up to 3A and if it needs more power then it should use higher voltage to reduce current below 3A.
So all devices up to this point will work like this, they cannot request more than 3A at any voltage and the cables are rated to pass only up to 3A and the maximum voltage you can request is 20V for up to 60W of power.
Then a new version of a standard gets defined.
The 20V was extended to 5A to provide up to 100W instead of 60W, but 100W or 5A is only available at 20V, and to overcome the requirement of 3A limit to 5A safely, it requires an e-marked cable so you cannot go to 5A unless your cable has an ID chip which says it is safely built to support 5A, otherwise the cable is the limitation for the whole system to use only 3A through a 3A cable.
And when the standard was expanded beyond 60W, it did not change the behaviour when the power is 60W or less - it's max 3A only.
If what you suggest was possible, to draw 12V at 5A, it is hard to justify why your device would want to draw 60W with 12V and 5A for 60W, because it will require anyway all the devices including the cable to be new enough to support the 5A 100W standard version, while you can already draw 60W at 3A and 20V with the old standard for less losses.
What I am trying to say, if you already have millions of people already using the old 3A standard up to 60W, why change any of that how it works as the USB is already complex for normal people to use.
The most simplest solution is, if your device needs 60W at 12V 5A, you transfer the 60W from any supply like Type-C with 20V and 3A and use a buck converter to bring it down to 12V 5A.
