How can spam still exist?

Shashank writes: "I have just joined a new company to manage their anti-spam product. This is the first time that I tried to dig out the figures from various research papers on the Internet. This is also my first experience with anti-spam market. I am suprised to see the competition. Almost everyone in the world seems to have an antispam solution. It is confusing that spam has not reduced in the last few years, more than 90% of the mails that any average individual gets is still spam.
If the available tools are so powerful, why is it that the problem of spam hasn't ended? I am not from an IT background. All that I have learnt about spam is from internet. I learnt that the spammers change their IP addresses and spoof using botnets. Is it not the responsibility of the ISPs to ensure that the users in their network do not get any spam?
Who should be responsible for customer privacy invasion, phishing attempts, junk mails in totality..?"