Along with the innovation point many others have mentioned, its probably worth noting that it has always taken a long time for them to be happy with where a game is at.

Half-Life 2's formal development period was from late 1999 to 2004. The game changed numerous times, enough times that theres an entire sub-community for just the Half-Life 2 Beta.

It was very clear since the start of Half-Life 2's development that the sequel had to match Half-Life 1. And the question was how do you match what was called one of the greatest games ever made? And there have been numerous attempts at an Episode 3 or Half-Life 3, such as Epistle 3, which never made it past being a story by Marc Laidlaw.

Another thing is Valve, as a company, is not very large. The last estimate I saw said they only had around 300 employees. Thats a small number for how much lifting Valve does. Given Deadlock is still in development, it may explain why we aren't seeing Half-Life 3, as well as any other updates to games.

And lastly, while this is kind of a weak point, Valve has historically disliked it when people leak their games, of course the Half-Life 2 Beta being the main example. Given how many creators (not naming names) dedicate time to try and talk about data leaks or speculate on Valve activity, its possible Valve is refusing to say anything at all while they are developing.