Then everyone would be receiving 100 spams every day. Theft of resources. An increasing number of spammers send most or all of their mail via innocent intermediate systems, to avoid blocks that many systems have placed against mail coming directly from the spammers' systems. This fills the intermediate systems' networks and disks with unwanted spam messages, takes up their managers' time dealing with all the undeliverable spam messages, and subjects them to complaints from recipients who conclude that since the intermediate system delivered the mail, they must be in league with the spammers. File it in the trash.
Most spam messages are in no doubt worthless.
It's all stuff that's too cruddy to be worth advertising in any medium where they'd actually have to pay the cost of the ads. Also, since the cost of spamming is so low, there's no point in targeting your ads, when for the same low price you can send the ads to everyone, increasing the noise level the rest of us have to deal with. Spam software invariably comes with a list of names falsely claimed to be of people who've said they want to receive ads, but actually consisting of unwilling victims culled at random from Usenet or mailing lists. Spam software often promises to run on a provider's system in a way designed to be hard for the provider to detect so they can't tell what the spammer is doing. Spams invariably say they'll remove names on request, but they almost never do.