Well, it's not exactly hard, but it does require a fair bit of knowledge (Unix/Linux, Apache, chroot environments, PHP/Perl, MySQL, SMTP/POP3/IMAP, TCP/IP, DNS, DHCP, and so on, and so on, and so on) plus a 24/7 standby crew.
As Thanee said it's a lot of work and even more money to get started.
That said there are plenty of places that let you be a reseller for an existing hosting companies. Could be less risk, but the rewards are not as great.