First, history plays a key role. The credit rating system started out this way (with letter ratings and modifiers) decades ago, and since then so much national legislation, international regulations, and corporate policies have been crafted around the existing system that it'd be very costly to change. For example, BBB- or higher is the legal definition of "investment-grade", and many financial institutions (insurance companies, pension funds, etc.) are legally barred from investing more than a certain percentage of assets under management in non-investment-grade securities. Similarly, Basel III and national reserve requirements assign different risk weightings to different credit rating levels -- AAA and AA may have a zero weight, for example (no capital is required to be held against the possibility of default for these classes of securities), while high-yield investments below C may have a 50% risk weight.
There actually is one rating agency that does use a 0-100% scale, but their scale is actually more difficult for the people who actually use the ratings (fund managers, policymakers, chief risk officers, etc.) to understand since it does not correlate as directly to existing regulatory and legal definitions.
Second, there actually are loss-given-default ratings like you describe, but they are assigned to specific securities rather than to companies as a whole. In fact, there are actually many different types of credit ratings. The one you hear most often is the long-term corporate issuer (or sovereign issuer) credit rating, but there are also short-term ratings, foreign-issuer ratings, loss-given-default ratings, etc.
A company would typically have many of these ratings simultaneously -- e.g. a Canadian company may have an AA rating for CAD-denominated short-term bonds, a A rating for Canadian-dollar-denominated long-term bonds, and a BBB- rating for US-dollar-denominated long-term bonds. Moreover, although the company's US-dollar-denominated long-term bonds issued last week were rated only BBB-, they have a loss-given-default of only 1% because they are structurally senior in the capital structure to the rest of the company's debt, whereas the loss-given-default rating for its AA-rated short-term debt issued yesterday may actually have a loss-given-default rating of 85% because it is subordinated to ten other bonds.