Internet attacks are easy because Internet users place unwarranted trust in the network. It is common for sites to be unaware of the amount of trust they actually place in the infrastructure of the Internet and its protocols. Unfortunately, the Internet was originally designed for robustness from attacks or events that were external to the Internet infrastructure - that is, physical attacks against the underlying physical wires and computers that make up the system. The Internet was not designed to withstand internal attacks - attacks by people who are part of the network. And now that the Internet has grown to encompass so many sites, millions of users are effectively inside.
Internet attacks are easy in other ways.
Technically competent intruders duplicate, share, and package their programs and information into user-friendly form at little cost, thus enabling naive intruders to do the same damage as the experts. Information on the Internet is transmitted in packets, each containing information about the origin and destination. A packet can be compared to a postcard - senders provide their return address, but they can lie about it. Most of the Internet is designed merely to forward packets one step closer to their destination, with no attempt to make a record of their source. Moreover, the Internet is designed to allow packets to flow easily across geographical, administrative, and political boundaries.