Fundamentally, it goes like this...
having made enough money to buy a nice big and/or historic house from your software company the choices go like this
a) buy nice big and/or historic house
b) buy another software company. The integration will be a failure, the person selling it is probably doing so because they can see that the business is on the way out, and they want someone gullible to give them some money so THEY can buy a nice big and/or historic house
c) invest the money in another product. According to the wise words of Nathan Mhyrvold, most software companies make their money on one product and then piss some or all of it away on others. Beyond the difficulty of coming up with the second good idea, by the time you come to do the second product, everyone will have become older, more pompous and more complacent.
Frankly, it is easy to see why people opt for (a).