Valve is definitely a victim of the hypetrain that Half Life 3 has created. Remember the hype for Duke Nukem? Remember how that turned out?

On a smaller scale, I definitely think Fallout 4 was a bit of a letdown after that whole hypetrain happened.

If they release Half Life 3 and it isn't the transcendentalist, holy experience that everyone expects it to be, then people are just going to be frustrated and disappointed. So they'll just not do anything. Better than crushing peoples' dreams with a reality that might not be up to par.

TLDR; people are in love with the idea of HL3. Maybe not the reality.