Modularity and separation of concerns, in hardware. Historically, you wanted your PIC or 8051 to be in complete control of the system. You built the core of yo

Modularity and separation of concerns, in hardware. Historically, you wanted your PIC or 8051 to be in complete control of the system. You built the core of your RTOS around some well-understood central processor, but farmed out tasks like converting text display over an SPI-like interface to modulating a 16x2 character LCD, or decoding your UART into USB serial to an FTDI chip, or decoding MP3 bitstreams into I2S or raw DAC signals, or what have you. SOCs added on-chip peripherals for some functions, but a lot of stuff was off-chip like in this Arduino. You wouldn't run code on an FTDI chip, that's an inversion of the architecture, and back in the day that was an ASIC that did nothing but covert RS232 into USB packets so there was no way to run code on it. The ESP32 is what it is because decoding 802.11 RF signals and running a TCP/IP stack is now a task not for an ASIC but a generic microcontroller that's fast enough to do those tasks in software. The processor is so much more powerful than the ancient Atmel Atmega328 in an Arduino Duemilanove that had 32 KB Flash and 2 KB SRAM on an 8-bit bus at up to a whopping 20 MHz that it seems ridiculous to do anything at all with such a 'master' processor.