The reasons are
1. The battery does act as a spike reducer and reduces ripple, some battery chargers have poor ripple control.
2 . A good power supply has good step response characteristics to load changes this prevents under or overshoot. In my experience since the battery acts as a damper, some chargers have very poor control of step responses without the battery in circuit.
3. Batteries act as a good low impedance supply, many charging circuits have poor current
sink abilities and handle load dumping transients badly, this can induce significant spikes as high current
. Some chargers in the absence of batteries raise the voltage to absorption levels, this can be upwards of 15 volts, over a longer term this stresses certain electronics designed for an average battery voltage. Some software
"hunts" for the battery changing voltage levels.
Yes some specific manufacturers have designed the chargers to work as power supplies. Though I don't think I've seen one actually say it can be used with the battery taken out of the circuit , your milage may vary.
If you have a long term need for a good DC power source, which excludes a battery in circuit , but a switch mode power supply unit, cheap
as chips these days.