Because the companies know from experience that once they have a customer, he will normally buy something else. That buyer of the new software package will probably buy the upgrade to it later on. Companies talk in terms of the 'lifetime value' of a customer. This can be many times the value of the original sale. More people have gone bust in the 'mail order' business trying to make money from a single product than in any other way.
Email marketing runs along a similar vein and the guidelines for success remain the same.
Lesley Anne Lowe is publisher of Advance Consultancy News, a free email newsletter geared towards online business promotion, developing powerful marketing strategies and maximising online profits.