Porcelain behind teeth?

I have fairly thin teeth. I smoke and drink red wine. I get my teeth de-tarred every year or two. I hate it when my teeth look grey...not because of the front of them...they look fine...its because the back get black from tar, etc.

I have an old porcelain tooth mid-back that has NEVER got tar stains upon it...its the same color as when I got it 20+ years ago.

My question is....if my teeth look good in the front, nice shape and color when the backs are cleaned...why dont dentists offer to thicken the BACK of the teeth with white stuff (porcelain?)so that the darkness doesnt show through or so that the tar doesnt adhere?

Is there a reason porcelain wont cling to the back of the tooth as opposed to the front? If I had icky front teeth I'd go for veneers but they are lovely when the back of them is cleaned....so why cant they glue something in back so they look clean regardless and enable flossing still?