Why we need to source package in order to use rosservice

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I've implemented very simple node that contains service to reset a counter. When I open up a new terminal and perform the following

rosservice call /request 'reset'

I get this error:

ERROR: Unable to load type [srv_counter/CounterServiceMsg]. 
Have you typed 'make' in [srv_counter]?

Even though the node that contains this service is running and rosservice list indeed shows the service in the list. Sourcing my package source ./devel/setup.bash solves the problem. I'm wondering why this occurs. Why rosservice is able to see the service but fails to call it.