Because a PCB based heated bed is cheap
Aluminum heated bed costs 4-5 Euro, already cut and you only need to drill 8 holes for the support.
The surface is electrically conductive, that makes it perfect for the auto-balancing mechanism (if the 3d printer supports it).
NOTE:
you have only to choose "untreated" aluminum plates, there are some with a protective film on the top, and this protective film is not electrically conductive.
and easy to construct
as easy as aluminum, but, to do a proper job, cutting PCB requires special tools, you can cut it with a common cutter, but after a while, it destroys the blame since the PCBs are made of glass powder and it's abrasive.
just like etching any other PCB, just a fairly large one
but the copper does oxidate, and you need to protect it. How to do so?
If you make a heated aluminum bed, you have to heat the aluminum somehow
the heater is a component, it stays below the aluminum plate, these parts are in contact, and the heater transmits the heat to the aluminum plate that diffuses it on the printing surface.
Thus you mechanically have a heater plate, and you have the aluminum plate on the top.
They are blocked with screws, and it's very easy to be assembled.