Each single-purpose system requires separate power, processors, memory and management. As a result, these WAN functions cost more than they should, driving up the price of each fully managed WAN connection by hundreds or thousands of dollars. Yet all the data feeding these systems must traverse the router as it performs the relatively simple task of passing packets. Sure, many routers offer extended features, such as circuit multiplexing, compression, NAT, tunneling and basic firewall protection. This is a good thing because these particular functions must alter packets to do their work, adding latency.
Every microsecond counts when packets are being compressed, encrypted or otherwise modified.
A single development source incubates merely adequate software. You'll still find standalone firewalls, VPN systems and compressors with more functions and better performance than their router-based counterparts. If this situation were going to get better, it would happen in one of two ways. This would give users a choice of products that serve a given function, and along the way add value to the underlying platform. It may sound Utopian, but it's common practice in the voice world, where third-party developers have long been embraced.