Why I have to replace my broken iPhone glass with Apple?

You don't, you warranty is long since expired. But you will not be able to get an Apple display unit and you need to go to a competent 3rd party shop to make sure they do not replace your home button nor disconnect it. And Apple may refuse to service your iPhone if something goes wrong.

You don't have to. I've had aftermarket repairs done to Apple devices outside of the United States where it was much cheaper and the job was satisfactory. There are no guarantees though. An at least with an iPhone 6s there won't be those nagging messages about there not being a "genuine battery" or "genuine display". But it's up to you. Official Apple repairs are generally at a high standard and have fewer problems.

If you want the replacement fixed correctly without voiding the warranty or damaging the product.

Check ▷ AppleCare Coverage Here.

Then ▶︎ Get Screen Repair Here.