> Web page creators: It is not your job to make sharing easier.
Actually, it pretty much is. That is one of the key ways in which sites gain audience, and it's a huge topic of concern among website creators, owners, and programmers.
Shares drive traffic, and both the website owner and the social network want you to ask yourself on each shareable page, "Is this something worth sharing?" Some people will say yes and that's a win for everyone.
You can say the on-site button doesn't do anything, but it really does. It sends a signal: Other people think this content is worth sharing, you might think so too.
There are other models for sharing content, for example a site only needs a few self-interested posters on HN or Reddit to get traction there, then the voting system and "front page effect" take over. But for a decentralized social site like Facebook, you need a good number of seed posters to decide, "It might be in my self-interest to share this." That only happens with prompting.