SBC is a PCB which includes ports for power, I/O, often other parts like sensors on it. Esp32 is a MCU. You can put an Esp32 on a PCB and call it an SBC. (Although semantically SBC usually refers to something that runs a GPOS, but perhaps that's flexible or changing)
Ex - go check amazon, they're all the "dev kit" versions of ESP32.
Hell - even Alibaba has an overwhelming majority of the kits in the SBC form, rather than the straight MCU (although they at least exist in that form there, unlike amazon).
Even on digikey... there are like 4 listing for the straight MCU, and hundreds of listings for PCB board versions.
So I think for most consumers purchasing these... functionally they end up buying a form factor that is much closer to SBC than MCU.
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From my perspective... I'm old enough to remember when Arduinos were the hot shit in this space, and now ESP32 is basically the same value prop at 1/10 the price point (or better, if you are actually willing to solder it to the PCB yourself... then you get down to 1/50th the price point).
Last time I checked digikey had every module version avaiable for every version. You get hundreds of other versions because there are more variations of those.
1. Go to mouser.com
2. Type ESP32 into the search bar
3. There's your part.
Its difficult to buy a very specific nut or bolt from Amazon (their nut/bolt search interface doing precisely nothing it appears to be intended to do). That doesn't mean McMaster-Carr doesn't exist. You're just going to the wrong store.
If you're in particular referring to how hard it is to find a PDIP package (that is, a through-hole package) for an ATMEGA MCU, that's because they don't make them anymore. Insofar as I'm aware, Espressif has never made a through-hole ESP32 module, but maybe they made one with castellated edges?
But an ESP32 module is as easy to come by as a surface-mount ATMEGA328P. Which is to say, very easy.