Why should I be expected to tip one and not the other?

If you want get into the long history, alcohol service is unique in that it places liability on the server rather than the business. This means that they are legally their own independent agent and as such are free to charge their own rates. The tip separates what was sold by the restaurant and what was sold by the server. You will notice that any old laws on the books about tipping refer specifically to alcohol.

This is why it has been traditionally customary to tip in establishments that sell alcohol, but not places that just sell hamburgers. But these days I'm pretty sure everyone will ask you for a tip and a lot of those old laws have been stricken from the books. It is of little applicability these days. Now, it's mostly just because people enjoy tipping.

Also, why is 15-20% considered a proper tip?

Because that is what the market will bear.

10% shouldn’t be treated like it’s an insult.

So what if it is an insult? I think most everyone feels a little insulted when someone says "no" to their offer. But are you going to accept every telemarking scam and vacuum salesman just because you are afraid you are going to hurt their feelings?

Lots of people get pleasure from tipping. And if a server can provide that service to them, great. If that is not you, just don't pay for the service. You can't win every customer, as they say.