The reason the default values are not saved in the TMX file is to avoid overriding the values defined on the object type. As such, if you’d ever change the values on the object type, any objects for which you have not explicit set other values will automatically take on those new values. This is often desired, because copying the base value all over the place takes additional storage (can be quite a lot if you have a lot of properties and object instances) and it makes changing the default very cumbersome (need to update all occurrences).
Additionally, when I introduced the object types I considered that in most engines, the properties would already have hardcoded defaults. In that case those values in Tiled would reflect what is hardcoded in code, and they would not need to be actually loaded by the game.
However, I understand that in some cases it is useful to define certain non-hardcoded types, but that at the same time you would not want to code another reader for the object types XML file (something you could do however, since you can put that file in your game folder). Recently, I have added the option to resolve the object types and custom properties upon export (not on regular saving). This way, you will find those defaults are also saved to the map file, as long as you use the Export action (and I would suggest you export to a separate file than the file you usually save to - possibly to automate this process with the command-line