This is Motion's way of making sure that a re-export of the project is distinguishable from previous exports in case you have made any changes. You can however name the export in the last window before clicking save. If you want the same name just change the name to the previous export and Motion will give you a warning to replace the earlier export, Most apps overwrite the document when you choose save and you have to save as in order to not overwrite so Motion's approach is more protective. I know of no way to change this behavior.
Also, in that last dialog in the export process, if you click on the name of the previous save (even if it looks dimmed,) the name of the file you click on will replace the name of the current file being saved. Then click Save and accept replacing the former file.
You can click on any file name that appears in that dialog and the name you click on will be given to the file being saved. This is NOT a Motion specific shortcut. It's part of the OS. It's really convenient if the new file is Untitled, or in Motion, is a recovery file.
Of course, clicking the file to get the name filled in the dialog box is something I've been using for ages, but then I need to scroll trough the folder each time to find the prev export. And renaming the file is an extra interaction, thus, not as easy as simply hitting ok on the overwrite dialog.
I just wished there was a preference setting for that.
It seems that there are more lost features in Motion5, like Export Presets (separate from Project Presets), or UI presets... eg. It annoys me so much that each time I have to scroll the list of the Library pallette to see the Favorites folder. I might be nagging about these silly things, I know :-)