I don't get why people keep saying that when
most HL games aren't revolutionary by any means.
Only one game was impressive on a technical level and that was Half-Life 2. The first Half-Life ran on GoldSrc and didn't really do anything impressive tech wise, in 1998 it already looked worse than Quake II released in 1997 and in 1998 it had to compete with Unreal that was actually praised for its' tech. Half-Life Alyx wasn't revolutionary either because it was released after
BONEWORKS which did everything that Alyx did, except 3 months earlier.