The principal reason is the complexity of integrating ever-changing legalities from all three branches of government across all jurisdictions – and it's not just the logistics; it's the very modality of interoperation with democratic institutions. Despite a modern push for efficiency in public administration, extant inefficiency is still deep and vast, and capital markets can hardly bear it (i.e. investment in development of the concept).
But don't discount a secondary complexity – that of the contextual analysis and pure judgment inherent to legal practice, for which the human mind has a mysteriously unsurpassed capacity.
Proprietary tools that do those sorts of things are currently penetrating the market, yet slowly. I've been called to advise on a few projects under development, but have found the mindset behind them lacking for meaning or purpose beyond commercial activity, which is perfectly ok and useful – though what interests me most in this realm is version control coupled with open access and put to use by citizen groups for legislative proposals.
Check out Contract Express from Thomson Reuters (http://contractexpress.com) for the establishment's instantiation of something that approximates the features you describe.