They just happened to have the foresight to develop the most lucrative business model in gaming (owning the distribution platform) and Half Life 3 was the opportunity cost. They could have made a ton of money with that cliff hanger if they remained like every other game developer on the planet, instead of a metric buttload of money.
Opportunity cost? Steam was created shortly after half-life 1 and existed prior to half life 2's release. It was already a huge cash cow when they released games like CS go and TF2.
But further, the skills to write a game are different from the skills to manage a game distribution system. It's not like by having the HL2 devs valve was squeezed for cash or resources to expand steam.
I think it's pretty reasonable to assume if Valve never made Steam, they'd have made Half-life 3 do you disagree? Businesses tend to focus on what moves the bottom line most dramatically.