Injection-moulded plastic is inexpensive to produce in volume, and gives many options in how to design it. CNC-routing aluminium is more expensive, both in cost of materials, machine time and wear of the different bits. It is more difficult to route certain details also.
Plastic is also a material which is room-temperature to the touch.
There are rubber dome keyboards where the bottom half is sheet metal (BTC...), and there are mechanical keyboards which integrate a sheet-metal switch mounting plate with the top case (Corsair...).
Metal keyboard cases however have to milled from a solid block. This gets very expensive very quickly.
Which is why Apple makes their keyboards from the leftover bits from making iMac cases. The aluminium shavings are washed and recycled, which also recoups some of the cost of the original blocks.
Apple's manufacturing process is very streamlined.