Because a physical lid is ugly and inelegant.

Dell elite books have very elegant camera shutters with a barely visible slide.

But obviously this is beyond Apple.

So do their flagship Precision series laptops. Dell knows how to cater to their professional user base.

I happen to have (collecting dust somewhere...) one of those old Firewire webcams Apple made. It has a physical shutter you can open and close by rotating the front. It's about as beautiful and elegant as it gets.

If Apple still cared about beautiful and elegant products, it could surely find a way to incorporate a miniaturized version of that in a Macbook.

My Thinkpad comes with a physical lid that works perfectly and is in no way inelegant.

My ThinkPad comes with a butt-ugly lid which works perfectly, and I don't care about the ugliness.

Tape it is then.