Because it's phony. If you want to do commercial product photography, pretending like you do the work already makes you look unprofessional.
I'm a graphic artist. If I had a bunch of tear sheets of ads and screen grabs of work all over my website with big corporate logos on them, it would appear like I'm not confident enough in my own work or my clients to display the work I do for them. It would also look really terrible when someone says "Oh, wow, all that work you did for verizon was cool. I've never seen that ad / site / commercial before". How are you going to explain that?
If I was considering hiring a photographer for a job that I was working on, and in looking through his/her website I noticed something like that, I would probably ask them about it. If they said "well, I didn't actually shoot those photos FOR brand X", that would likely be the end of the discussions for me.
If you want to shoot product photography, and that's what you want to display, then shoot photos without logos. Peel that label off and light the glass in a cool way, or display it with a colored liquid inside.