Sometimes there are no other option other then selling your company.
You can become outnumbered in the board and become a one men army due to different views of the future or other complications ( try that... it's fun like hell). You can work for your company for 40 years and realise YOU are not needed anymore and you should sell it and enjoy your last years.
And so many other variations to that "Why?" question. I found that statement aimed to make people that sold their company look like they gave up. And more often then not. This is not the case.
Thinking like that is just to simple.