If that were the case and the camera makers raw software gave so much better quality conversions then everyone would be using it. But you only have to go on the photo forums including here and what you hear most to the question which raw converter do you use is Lightroom or CS4!
That's not entirely true of course, or at least my understanding/interpretation of what I have been reading on various forums is somewhat different. My understanding (and also personal experience) is that Canon's own software (DPP) produces far better results straight out of the box when compared to Adobe's ACR. Where it does fall down is the 'extras' provided by ACR such as the TAT and Brush tools (to name but two) which makes using ACR/Photoshop/Lightroom overall far more convenient. DPP has become increasingly better over the years and if Canon would/could only expand upon it a bit I feel sure even more users would remain with it. Let's face it, and taking only the RAW converter into consideration, when one considers what one pays for Photoshop (ACR) when compared to the FREE offerings of Canon it really is no contest.