Why? Because they need developers to make worthwhile apps. Yes, the original iPhone shipped without an App Store, but it could also do regular phone stuff (calls, sms, etc). What’s the “regular” stuff you’d expect to do on a VR headset? Other than watch movies on a big screen, there’s currently very little reason to own one, and encouraging everyone’s mom to buy one is setting them up for failure.
Give it a couple years for there to be some really groundbreaking apps, and Apple will open the floodgates with a cheaper/more ergonomic version.