A ticket allows you to travel along a route, usually on a train of your own choosing.
So a ticket for Zurich - Geneva usually allows you to board any train from Zurich to Geneva.
That is why it does not come with a reservation. The railways do not care when you actually plan to travel. You are free to go at your own discretion.
A reservation is almost always optional, usually unneeded (most people here would not bother) and gives you a seat on one particular train. Al it does is guarantee that you will have a seat on that train. But you can board the train without a reservation (as most people do) and just look for a free seat yourself.
That is why they are separate. This is how trains work…