Of course people are going to imitate their favourite bands when it comes to making music. You can hardly expect people to make music they don't like, can you?

The problem with your argument is that you're working under the misapprehension that an imitation can never match up to or surpass the original. Taking Oasis as a popular example, they stole everything but their name from The Stone Roses, up to and including having an arrogant, tone-deaf moron for a lead singer. Despite this, their first two albums are widely accepted as being classics, and easily on par with The Stone Roses' work.

jrgen is also correct in saying that bands take on a number of influences in their formative years, and that this has the effect of varying their sound. To use your example of Metallica: just how much do they have in common with Led Zep and Hendrix? You'll pick out a few similarities, yes, but the overall sound is really quite removed, the reason being that every member of the band brings multiple influences to the table. You'll be hard-pressed to find a group of musicians with identical taste, so any band will inevitably end up assimilating the styles of more than one group, as well as adding their own input into the mix.

Besides, it's not as if all the great rock and roll legends weren't imitators themselves. Led Zeppelin owe much to the blues; The Beatles started out playing the sort of simple rock n' roll that had already been popular in America for some time (Elvis, anyone?). Sometimes, because of the legendary status of these artists, it's easy to forget that they were/are music fans themselves, and that they wanted to be like their heroes as much as any band of today wants to be like them.