valve tweaks/updates it with every new release (just compare HL2 to portal 2)
also the Source Engine is based partially on the goldSRC engine (used in CS and HL1/OPFOR/BS setting it,s roots somewhere late 90,s) it,s physics are done by the Havok Engine.
PS
also Valve,s games are more varied (instead of just playing a random army guy) you play a rebel in HL2 and the episodes (I know Gordon is a scientist but his action make him more a rebel) a test subject in Portal(2) and a insane mercenary in TF2 it also keeps using different environments per game further hiding the fact it,s a old engine.
PPS
there are also benefits to sticking to a older engine
for example porting the game would be faster and you don,t need to learn new things when developing a new game allowing games to get done faster.
also the Source Engine is based partially on the goldSRC engine (used in CS and HL1/OPFOR/BS setting it,s roots somewhere late 90,s) it,s physics are done by the Havok Engine.
PS
also Valve,s games are more varied (instead of just playing a random army guy) you play a rebel in HL2 and the episodes (I know Gordon is a scientist but his action make him more a rebel) a test subject in Portal(2) and a insane mercenary in TF2 it also keeps using different environments per game further hiding the fact it,s a old engine.
PPS
there are also benefits to sticking to a older engine
for example porting the game would be faster and you don,t need to learn new things when developing a new game allowing games to get done faster.