There are many reasons but here are THREE of them!
I'm going to cover the top three here along with short explanations:
1. Plug in headphones
If you don't want to disturb others, this is a biggie!
2. Never needs tuning
The instrument can save your wallet AND your ears.
3. Learn MANY instruments
No need to buy and then learn all those instruments. Simply dial them in on a keyboard!
No other instrument gives you all these advantages - except an electronic keyboard.