My experience: because you can probably still do it better.

Google, Facebook, and Twitter are far from omnipotent. Want proof? Just look at Google's failed attempts to get into TV advertising or social networking. Or Facebook's attempts with Messenger. Or Twitter's... everything, really.

Also, consider, it's unlikely any of them would copy an idea until you've already made it a success, at which point network effects can work in your favour (again, Google's repeatedly aborted attempts in social networking are illustrative, here).

To summarize: they might try to copy you, but there's no reason to assume they'll succeed.