<p>Anyone visiting a torrent tracker is sure to find droves of "cracked" programs ranging from simple shareware to software suites costing thousands of dollars.  It seems that as long as the program does not rely on a remote service (e.g. an MMORPG) that any built-in copy protection or user authentication is useless.</p>

<p>Is it effectively not possible to prevent a cracker from circumventing the copy protection?  Why?</p>

Copy-protection is like security -- it's impossible to achieve 100% perfection but you can add layers that make it successively more difficult to crack.
Most applications have some point where they ask (themselves), "Is the license valid?"  The hacker just needs to find that point and alter the compiled code to return "yes."  Alternatively, crackers can use brute-force to try different license keys until one works.  There's also social factors -- once one person buys the tool they might post a valid license code on the Internet.
So, code obfuscation makes it more difficult (but not impossible) to find the code to alter.  Digital signing of the binaries makes it more difficult to change the code, but still not impossible.  Brute-force methods can be combated with long license codes with lots of error-correction bits.  Social attacks can be mitigated by requiring a name, email, and phone number that is
part of the license code itself
.  I've used that method to great effect.
Good luck!