Because it works and it's easier than dismantling the ship to get to the fusion reactor from the outside whenever a major reactor overhaul/replacement is required.
At some point a ship is going to have to dock for a mid-life fusion core replacement or service. When that time comes you can;
A) Waste time and money having the dockside engineers dismantle a section of your hull work and then work 'inwards' to get to fusion reactor itself; or
B) Once docked and hooked up to the repair stations power and life support systems (and once the engineers are ready initiate a reactor core 'ejection') all interlinks between the ship proper and its fusion reactor are disconnected internally and then the core is 'ejected'. Not as violently or quickly as the old fission models were ejected mind you because that is not required.
This is not a high speed, life or death emergency like an 'old school' fission core meltdown would be. Instead in this case 'ejection' simply means an internal mag rail system gently pushes the core rearwards out of the ship, like a chicken laying an egg - strait into the mechanical arms of the waiting docking portsport's waiting capture cradle. (Reverse process to install newrefurbished/new reactor.)
Long story short? Its still called 'ejection' because that's what happenshappens but it's not the same procedure and not conducted for the same reasons as a fission core ejection was or is. Spacers being spacers however just stick with the terminologies they know.