entries quickly enough. When you reboot a router, all the routing tables are cleared out because all memory is cleared without electricity.

An entry can be removed based on time stamps, and if you don't have any entries that are past the time stamp expiration date in the tables, the code won't remove them yet.

This is the most common reason, but not the only reason. If you have that problem a lot, you probably need to get a router with more memory or the ability to configure how old a route needs to be before it gets removed from the routing table.