firmware does not have the overhead of a host OS. super low power modes, instant sleep/wake, exact control of instructions/clock, far fewer security issues.
Teensy and ESP32 are just souped up Arduinos.
Cheaper, more powerful, more feature-rich Arduinos, if you squint. I'm not sure why you would actually use an Arduino™ unless they've really improved their lineup since I last looked.
The clones are so much less expensive than the official Arduinos that there's no comparison. Even the versions that Adafruit makes that IMHO, improve the design cost less than the official arduinos. So it's not a surprise that the Chinese clones will be dirt cheap. I'm not complaining though. I have a board where the Nano (clone) itself is by far the cheapest "chip" in the BOM. Precision analog devices are expensive and my 50 or so boards sold each year can't compete with the Shenzen manufacturers' economies of scale.