The reasons could go on for days, but the biggest reason is that you can reach more people with less effort than traditional sales and marketing.

I am a life long sales person. I have traditionally sold high ticket retail items. Cars, boats and homes. What is nice about the Internet I can sell other peoples products world wide from my home. Using the world wide web.

Other than the reason given above here are some more good reasons:

• Low start up cost. All you really need is a domain and hosting to get started if you are serious about making money online.
• No customer service if you are doing affiliate sales.
• No schedule. I recently reentered to offline work force as an Internet Manager for an auto dealership, unfortunately I have to work a schedule now and I can't stand it.
• You can make money when you are not working. I make money from sources like AdSense when I sleep and from sites I have not updated in a long time. Granted this is only about $100 per month from this old content but it is nice. I don't have to work for it.
• No product handling. Even personal products I sell I don't have to handle inventory. It is all through digital delivery.
• Low to no advertising cost. If you really study your trade and keywords you do not have to buy advertising. You can let the search engines like Google and Yahoo bring you your customers. I do some PPC arbitrage however I really don't like to manage the campaigns. So if I am unable to monitor them like a hawk I turn them off. Nothing is sweeter than generating sales from the search engines.
• Time factor. I can spend 100 hours creating a niche site and submitting it to the search engines and start generating sales fairly fast. Then my only maintenance is updates to content if there is a change in the market for the product or I decide to switch affiliate offers.
• Freedom. If I have things pressing in the real world I don't have to ask for permission. I just do it.

Although this list is not all inclusive it gives you a look into the mind of the "mad affiliate marketer"