I think you have some very odd opinion regarding BT transport
As said before, making a BT transport is almost as complex and expensive as making a DAP, sharing most of the same internal component as well as software. To ask a manufacture to make a BT transport is about the same as asking them to make a DAP without screen and internal storage. So why won't them just make a DAP with BT transport capability (and in fact that's precisely what they choose to do)? That way they can sold the same device for multi functionality instead as an one-trick pony.
As far as my opinion goes, those DAP you pointed out are exactly the kind of BT transport you are looking for if you used them that way. Removing the screen and internal storage will not have any major impact of cost (*we are talking maybe $20~30 less per unit, could be even less), since most of the cost will be on the SoC and the firmware development.
On the other hand, I think most people who ends up using BT with higher end headphone are well aware about the shortcoming of BT impact of SQ,. Those people choose BT because they want the convenient and simplicity of having a very small device that can pair of a smartphone and willing to sacrifice some level SQ and output power in the process. An BT transport connect to a a portable DAC/amp in many way just defeat the idea of convenient and simplicity in the first place. For those who are willing to carry a full sized portable DAC/amp, we already have portable DAC/amp that has BT function built-in. I will argue that adding a small DAP with BT transport functionality to a portable DAC/amp will not actually impact most of those people willingness to carry a large stack of device on-the-go. So essentially a complex-to-build, BT transport only device is just a very niche product that don't quite fit into the market. It is an easy decision for the manufacturer - for about the same price, would you want to buy a small DAP with BT transport capability or just a BT transport? What do you think will be an easier product to sell? How would a manufacturer explain to its customer that the BT transport they made has almost all the hardware and software of a DAP, but they decided not to include a screen and internal storage to make it a fully capable DAP, but instead believe it is better off as a BT only transport?