Portal? University students that Valve hired.
Left4Dead? Turtle Rock.
CS:GO? A bunch of modders who Valve bought the game off of and ported it to a new engine.
Dota2? Another mod that Valve hired 1 guy to come in and remake for them in source.
TF2? Hey didn't Robin Walker make that? Before he got hired by Valve? In 1996? Are you seeing a pattern here yet?
Valve haven't shipped any games that weren't fundamentally somebody else's work which they've bought in over 12 years. And yet somehow they're important and relevant? No. Just no. Valve don't make games, they buy other peoples games and polish them loads and then try to pass it off as a new product. You know who else does that super successfully? Blizzard (if we ignore hearthstone).