I recall that there was an 8051 GCC, back when I was programming 8051 17 or so years ago. Actually I looked it up, it was called SDCC. The code was not too efficient and we went with the Kiel system as it supported the ST PSD that contained flash and SRAM.
It did not help that the ST PSDs were already pretty much EOL by the time I inherited the project. Ironically Some of the flash chips used to store the programs on were made by Atmel.
Supposedly the costs are cheaper to use the 8051, However the BOM on these boards is about 35USD compared to around 5USD for the mega328 version using off the shelf SD cards for the flash.