I think Shimano just builds way more 105 equipment compared to Ultegra and Dura Ace. The 105 big chainring is only a few grams heavier than Dura Ace. For the money, those extra grams can be cut elsewhere for much cheaper.
Also 105 Di2 groupsets are starting to appear on AliEx for really good prices. Ltwoo and Wheeltop aren't as compelling with higher pricing unless you really need 13 or 14 speed cassettes.