Because physics is already in end state. Sound didn't improve much from 1997 either. You can't make sound 2.0. Physics outside of total destruction already is in the game and total destruction is limited by game design not by processing power or lack of technology. Players could already destroy level in Red Faction before Half Life 2 was even made and yet Half Life 2 didn't include it.
Portal gun much like full destruction limits game design as you won't be able to have open environments, that's why portal games take place in those complexes.
No. HL2 wasn't the end game for physics in video games.
For example, you mentioned destruction. Why not have modern voxel or hybrid mesh systems to let players actually break down environments into realistic materials. Wood splinters, rebar bends, glass shatters dynamically etc.
Real time deformable materials. Metal bending, flesh ripping, fabric tearing.
True fluid dynamics where water, fire, smoke, and gas behave as continuous entities, interacting with objects and temperature realistically.
You said the gravity gun was "gay", but what about a new, improved gravity gun? A gun that can manipulate fields, not just objects. Bend gravity locally, change inertia, or alter mass dynamically. Imagine flipping a room upside down mid-fight or creating a localized zero-G bubble!
Think outside the box. HL3 needs to be as revolutionary as HL2. Not just a slight upgrade.