And the available chips (AVR, ESP32, LPC2xxx, etc.) are all proprietary designs with a single manufacturer. Even the ARM chips, which generally use some variant of a Cortex-M core, have wildly different peripherals. So migrating between families is difficult/impossible.

In contrast, "classic" chips like 8051, 6502, x86 and 680xx all have external parallel bus interfaces and are (or were) produced by multiple manufacturers (often as part of "second source" agreements).

So when building a system using these chips, the designer has a large degree of flexibility and freedom to design the system architecture. Whereas building something using modern embedded chips is mostly an exercise in parametric search trying to find an existing chip which offers exactly the right set of peripherals for the intended design. It reduces the system designer to a mere consumer of off-the-shelf SoCs instead of being a true builder/architect.