SPAM is defined as unsolicited commercial email. Not all SPAM are scams. Some SPAM is just a legitimate business' attempt to solicit business.
Running advertisements in local newspapers, magazines, or even websites is very expensive in comparison to sending emails. Marketeers are always looking for inexpensive ways to market their products. And just like Junk Mail that it sent to your home's mailbox, SPAM emails are sent to your Inbox.
And its a numbers game. If 1 person in 500 would buy your product, and you sell your product for $10, and you send your SPAM email to 1,000,000 people every day, then 2000 people will buy your product every day. Your business will have $20,000 in sales every day. Sending 1 million emails per day is very easy and costs almost nothing. $20,000 per day in sales is $7.3 million per year.
If you had a business and you believed you could make an extra $7,000,000 per year by sending emails, wouldn't you try it, at least once? And if the results were even half that, wouldn't you continue to try it? If you answer "Yes" then you know why people send SPAM.