I'm guessing it's a cost and complexity thing. More moving parts = more potential problems.
Separately, the HTC Vive sits over your glasses pretty easily, so it hasn't been a big issue. The Rift though... I have to bring out the contacts for it.
Moving parts are too expensive, I agree. But nowadays one can just 3D-print a set of lenses that will match one's vision perfectly, and making it possible to just snap them in and out might be easier to implement than any alternative.
But there is a good point there--Zeni optical probably pays under $4 per pair of lenses it sells. If you had a slot to insert a lens it wouldn't be very expensive.
Not as 100% of the process, but you can print a blob that is 90% of the shape of your lens and grind the rest of the material away as you would have to with normal lenses. I've seen people do it. It's labor intensive, but less so than starting with a regular lens blank.
Back when I still had glasses, I tried 3D printing a small structure that takes in lenses from an online glasses shop. It didn't quite work, and I abandoned it because my Vive is pretty decent with glasses.
Separately, the HTC Vive sits over your glasses pretty easily, so it hasn't been a big issue. The Rift though... I have to bring out the contacts for it.