Because only like 1 out of 5 tries, if that, can get the captcha right. XRumer bypasses captchas, but you gotta remember that not all of them are the same! YouTube's captcha is actually quite difficult for a program to depict each character. They are different fonts, sizes, colors, some have weird markings on the edges of the letters, placed in different corners. By the time you created 100 accounts, you would have half your proxies banned by YouTube for too many failed login attempts. If your proxy is transparent, you risk having your real IP banned. If you have any accounts created with that IP (legit or bot), those will be suspended.
So the short answer is, it really isn't practical for someone to attempt to automate youtube's captcha. In order for that, the coder would have to spend countless hours coding and finding a way to get YouTube's captcha right, majority of the time. That will jack up the price for that software. 1 because it would be the only one that can break youtube captcha, and 2 it took the coder 500 hours to freaking figure it out. Who wants to pay $500 for a youtube bot that wont work without a new update once YouTube changes things up again like they always do.
It just isn't practical for everyone, the coder nor the end user.
(btw I used to use privately released software such as TonyVegasOCR, OnCrackingRampage, and a few of the others in the xxx cracking scene. They each crack a different type of captcha, and you had to tweak it for every different site!)