And you really do "need" a special constant-current driver for high-power LEDs. This is NOT easy to build yourself!
LEDs are non-linear (like all diodes) so their resistance changes with voltage. When you feed the correct current with a constant-current power supply, the voltage "falls into place".
If you feed it a fixed voltage from a regular power supply the current is hard to control.
With "regular little" LEDs we use a series resistor to control/limit the current, but this is inefficient with larger LEDs and you need a higher-power resistor.
With a normal fixed-voltage power supply the voltage depends on load. With nothing connected no current flows but the voltage remains constant.
If nothing is connected to a constant-current supply the voltage jumps-up to however high the power supply can go as it tries to push current through infinite resistance.
In case you don't know this, high-power LEDs also need a heatsink.