Question: Why do summary routes get a lower AD than other routing protocols? For example: The AD of EIGRP is 90, whereas the AD of a summary route is 5.

Answer: 
Since the summarized route means that a router advertising it has knowledge of the individual routes within the summarized prefix, it is more trustworthy than the same (summarized) prefix being advertised as an individual route without the knowledge of the individual routes which make up the summary.

This doesn't mean that the summarized route is more preferred than one of the routes in the summary since the longest match will be more preferred than the summary. It simply means that the same route, if advertised as both a summary and a non-summary, the summary route is more trustworthy.
The AD of the EIGRP summary route is 5 only on the router that has the summary route configured. When the summary is advertised to other routers it has an AD of 90.

The reason for the low AD is to insure that the summary route (to nul0) is preferred to prevent routing loops.