In my experience reluctance to do demo's or lack of engagement with demo's results from a misunderstanding of what the demo is for.
It is easy for developers (particularly inexperienced developers) to think that a demo is a progress meeting. This happens when the focus is showing how much work has been done and proving the 'worth' of the team.
Demo's work best when the team sees them as an opportunity to get feedback rather than demonstrate progress. One way to achieve this is to encourage the team to phrase their demonstration as a series of questions:
This is how we built the sign-in page, what do you think of it?
The search box could have been in the menu bar, but instead we put it in the page body. Does that make sense?
The team talked a lot about the best way to display this table. What do you think of our solution?
This only works if the team values feedback and has the opportunity to act on it (i.e. the scope is not locked down).