Why in the world would a company making billions a year, for basically no work, take an enormous risk? And it isn't a financial risk, at this point next to nothing is a financial risk to Valve, but a goodwill risk.
EA and Activision, sure, they can take financial risks. Valve is at such a point that developing games is such a secondary income stream that no dev investment would be even remotely risky. Beyond that, they're private, and I believe wholly owned without an outside board, so they can do whatever the hell they want without answering to anyone.
And taking this another step, virtually anything with "HL3" on it will recoup dev investment and then some. It's probably the single safest development bet outside of a new Mario, new Zelda, and new GTA. Possibly higher than some of those.
No, it's a goodwill risk, in that it is essentially guaranteed to make the tiny little peabrained children that think they are entitled to the exact HL3 vision they have in their head freak out. CJ will be here whining about the same stuff he always whines about, every Valve game will get negative reviews on every platform, and any dev at Valve will have to delete their social media due to death threats coming in because they dared change something some immature asshole thinks was sacred.
Why would anyone mess with that? It isn't cowardice, as mentioned, it's guaranteed money.