Replying to question post in https://github.com/zsmartsystems/com.zsmartsystems.zigbee/issues/1226
That is really off-topic for https://github.com/zsmartsystems/com.zsmartsystems.zigbee/issues/1226 but as I wondered the same and have drawn a few conclusions and made some educated guesses based on their history and period product was developed based on when it was finally released. So I posted an answer to that as well as a rant about some related speculations here instead:
First of all, the short answer is that CC2531 USB dongles do not have a separate USB-to-UART Bridge chip (also known as a USB-to-Serial converter chip) but all CC2652 USB does because the CC2531 chip has an integrated circuit for USB-to-UART/Serial conversion and CC2652 chip does not, and neither does CC2530, CC2538, or CC1352 chips, so all those need an external USB-to-UART bridge/converter chip if want to make USB-dongle and not a serial-dongle/adapter. Simple as that.
What I think is probably much interesting story is why does ITead’s Sonoff Zigbee 3-0 USB Dongle Plus have a Silicon Labs USB-to-UART bridge/converter chip on its board and not a USB-to-UART bridge/converter chip from a different manufacturer, as the Silicon Labs CP2102N chip they went with is more expensive than most other alternatives yet does not really offer relative better features on paper.
Well, I think that for starters, the Silicon Labs CC210x is known for being very stable and already had mature device drivers on all operating systems at the point when they were developing the product. So from that point of view, the CC2102/CC2102N chip they use is an excellent choice, but otherwise, the only noticeable feature of CP2102N USB-to-UART Bridge got a writable on-chip EEPROM so they can write a custom product description. To get that same feature with a writable onboard EEPROM they could have alternatively gone with for example WCH CH340B or FTDI FT231 series which are less expensive USB-to-UART Bridge/Converter chips also has onboard EEPROM with similar features.
The benefit of choosing a USB-to-UART Bridge with a writable EEPROM is that it enables the manufacturer to write a custom product description string to the USB chip which is seen by the operating system, (you can even change the PID and VID if you like but that is often not wanted since require registering and releasing your own unique drovers). And that having a custom product description string to the USB chip allows applications to add automatic USB discovery which can give a more streamlined user experience for new users. So that is a small feature that can make the initial setup much more user-friendly. See example → Community help wanted to whitelist all compatible Zigbee and Z-Wave USB adapters for automatic discovery in Home Assistant OS - Home Assistant OS - Home Assistant Community
However we know that ITead’s first Zigbee 3.0 USB Dongle (and also there Sonoff ZBBridge Zigbee Bridge/Gateway) was based on the excellent Silicon Labs EFR32MG21 (Silabs EFR32 Mighty Gecko Series 2) chip, but since that dongle was sold very barebone and basic without an enclosure or retail packaging, (it seemed more like an experiment to gauge the DIY market interest in a Zigbee USB dongle than a real attempt to market a new product category), so my guess is that even though it had design issues it was still very successful and sold well so they must have wanted to make a new nicer premium version of a Zigbee USB dongle for the retail market using knowledge from the lessons learned from that previous dongle model and feedback/input from buyers and the DIY home automation communities.
Anyway, I therefore simply assume that ITead probably initially planned to use Silabs reference hardware design with Silicon Labs Zigbee MCU chip and a Silicon Labs USB-to-UART Bridge chip combination in it until the world was hit with the current worldwide chip shortage and especially stock of that EFR32MG21 became very scarce globally. EFR32MG21 has almost been impossible to get in larger quantities during the last two years, (though ITead is still using EFR32MG21 its Sonoff ZBBridge Zigbee Bridge/Gateway) product). Hence they probably switched from Silicon Labs EFR32MG21 Zigbee chip to Texas Instruments chip in the middle of the development cycle and did not bother to also replace the Silicon Labs USB-to-UART Bridge chip with some other USB-to-Serial converter chip.
PS: Mentioned before that as the ITead’s "Sonoff Zigbee 3.0 USB Dongle Plus sold at a low price yet contain premium components I assume that ITead hope to make money on attach rate sales instead of simply sales of this Ziglee USB dongle alone, so recommend those new to Zigbee to also check out some of their other Sonoff branded Zigbee devices as well → https://itead.cc/?s=zigbee&post_type=product&type_aws=true