Limiting IPs will not help
"Why don't they limit the number of accounts for each IP?"
Because the spammers are much more sophisticated than that. They "own" so many machines, it is sick. I run a mail server at my job/life. I have seen dictionary attacks that randomly attempt to send to thousands of users, where no IP is used twice. Every single email comes from a unique IP, for that attack.
With their massive network, they can create crippling ddos attacks on those who dare to oppose them. See http://www.securityfocus.com/news/11392
The spammers go up against reverse dns entries, residential IP blacklists, known spammer blacklists (spamhaus, etc), dynamic blacklists (spamcop), known sender "volume spikes" blacklists, rate limiting on the sender's isp side, rate limiting on the recipient side, port 25 blocking on the sender's isp side, greylisting, etc.. and they can still get the spam through.
They might send one or two spams per hour from each machine. But if they send from 100,000 machines, that is 1,200,000 to 2,400,000 spams per day. If they can use those 100,000 machines to open 10,000 live.com accounts per day, they might be able to send 100 spams per account, per day. adding another 1,000,000 spams per day to their tally, multiplied by how many days the account stays open.
I have seen spammer programs that check the rbl lists to see if the IP is listed in the rbls, before sending mail from that "owned" ip. Their side has some very ingenious programs, and some very ingenious programmers. It seems to be modeled after the internet itself, with a distributed control system (like how dns works) and hundreds of thousands of nodes, to distribute the work over.
Come to think of it, I wish that some of the regular software developers would adopt some of their tactics. Wouldn't it be nice, to have a database that has its content and meta data redundantly spread over thousands or hundreds of thousands of systems? With no single point of failure.
Wait a minute... strike that last paragraph. Replace with: I have an idea for a database that has its content and meta data redundantly spread over thousands or hundreds of thousands of systems (patent pending, copyright 2008, pending trademark 2008, pending service mark 2008). And data encrypted too (another patent pending, another copyright 2008, another pending trademark 2008, another pending service mark 2008)