Because HL3 would suck. First person shooters today are very different than what first person shooters were when HL2 was released. It's Quake vs Overwatch. So Valve is between a rock and a hard place; do they make HL3 in the modern style and alienate all their original fans? Or do they make HL3 in the dated style and risk a flop because the market for that dated style has only gotten smaller over the past decade and new players aren't interested in it? That's why HL3 would suck. No matter which path they take, and no matter how expertly they make it, it will almost certainly be something that leaves a TON of people dissatisfied.
So it's better for Valve to not make it. They'd rather HL3 be an urban legend than a disappointment. That's better for their image as experts in their craft.
So it's better for Valve to not make it. They'd rather HL3 be an urban legend than a disappointment. That's better for their image as experts in their craft.