At some point they will, but no one is going to update all the phones with iOS 7 that have been sitting on store shelves or in warehouses, so those ones will be sold with iOS 7 and new stock coming from the factory will have iOS 8 pre-installed.
If you're wanting to get a phone with iOS 7 for a jailbreak or whatever, note that it's quite possible that it may come with an older version of iOS 7 depending on how long it's been sitting around. If so, it will be impossible to upgrade to 7.1.2 because Apple is no longer signing it.
If you're just concerned about having to update to iOS 8 yourself, then don't worry about it... it's a pretty quick and easy update, especially for a new phone and in that case it won't matter which version of iOS 7 it comes with.