Why are transformers unregulated?
The voltage rises because the transformer has been designed for a specific load. If the load is removed the voltage will rise, perhaps by 50%. It's caused by the core flux being maximum with no load on the transformer. Current in the windings will generate an opposing field that reduces the flux, reducing the voltage.

Regulated transformers are called constant voltage transformers. Here is how they work. The additional winding and capacitor makes them more expensive than unregulated transformers.