A source close to Valve (these are our favorite sources, folks) has indicated that things have gotten pretty crazy for the potential world of Half-Life 3.
Next, instead of a pure FPS title, Half-Life 3 will feature a fully explorable open world with both puzzle-solving and RPG elements. Talk about changes. This is the bit we find a bit tough to follow. But, hey, stranger things have happened.
Okay, so, the other part that we believe? The source close to Valve indicated that Half-Life 3 has been under a "chaotic" development schedule. Yes. After this many years without any type of announcement or reveal since the last entry in the Half-Life series (Half-Life 2: Episode 2), we totally believe that development has been chaotic.
As for the other things? Well, it would certainly explain the time it's taken this game to be made. If Valve had a pure shooter and then decided to flip everything on its head in favor of open world elements, puzzle-solving and RPG styles, that certainly would have caused large delays.
Though, this is a rumor. Heck. Right now, there isn't even a Half-Life 3 of any kind.