I completely agree. Companies change over time. The startup you founded with one or two other people eventually becomes a business.
Now there are typically two ways of running a successful business: (1) culture; or (2) process.
If you build a business with a very specific culture that you will love to work within as the company grows then great. However building a business is much easier than establishing a culture you want that grows with the business.
Process is the alternative to culture as a route to long-term success and profitability. When you can't hire smart people that will maintain a company culture, you have to rely on process to scale. Even Warren Buffett once quipped, "I try to buy stock in businesses that are so wonderful that an idiot can run them. Because sooner or later, one will."
Once you take you business down the path to process instead of culture, you are creating a monster that hopefully craps cash. Eventually that monster becomes so big and powerful that your position as a leader becomes a position as a process manager.
Leaders lead people. Managers manage processes.
Processes don't have room for vision. Leadership does.
Once you have processes, you start looking for an exit strategy.
Besides the issue of culture versus process, many entrepreneurs also enjoy the act of creation most. These entrepreneurs are like artists. Once they've created something, they desire to move on to the next piece once they are satisfied with their current piece. The different between the entrepreneur and the artist is that for the entrepreneur it may take months or years before they are satisfied with their current piece, whereas the artist may take days to weeks to months to finish a piece and move on to the next.
Plus, deciding to move on to the next piece doesn't mean that you cannot stay on as an advisor to those who will continue to work on your previous pieces.