Why don't bikes by auto manufacturers make it into bike shops or onto the streets?
The simple answer is market forces. Most bikes done by auto makers are built as an add on to a vehicles' sport package. 

They are not manufactured or offered to bike shops at all. This tactic can be quite effective. Look at the VW Jetta Trek, or the Ford/Kona partnership. 

The problem is that they are generally very basic bikes, offered at a much higher price than the value of the bike. 

In the Ford/Kona case, it was a 500 US dollar bike offered as a 3000 dollar add on with the car. 
No reputable shop will do that. 

The Jetta Trek was a very similar package, but it was a $5000 add on and included a Thule rack for the car. 

Mercedes' offerings are a piss poor design, offered again at a ridiculously high price, although they at least have the excuse of a unique in house design and low production numbers to blame the price on, since they didn't just rebrand a cheap bike shop bike.