All commercial aircraft have checklists in place, and a two person cross checking system in the cockpit, as well as mechanical means to detect potential problems - but aircraft still every now and then land with their gear still up etc.

Does it make economic sense for an organisation that has invested possibly millions of dollars in training a pilot to immediately sack him/her for a costly mistake like this? Sure, disciplinary action and possible demotion within the ranks is a given, but will you get rid of the one person who will be almost fanatical about never landing with the gear up again?

If they consistently make technical mistake and are sloppy in their airmanship, then sure, fire away. But it is going to be pretty much impossible to design a 'system' that can prevent human errors from creeping in.