A brief explanation why…
After having problems keeping online games such as Counter-Strike up to date with new patches, Valve sought to create a platform that would distribute game updates and improve anti-piracy/anti-cheat measures. However, it's said that selling games on the platform wasn't even considered initially.
Speaking with GamesIndustry.biz back in 2008, Valve's VP of marketing Doug Lombardi said that the developer tried to convince "Yahoo, Microsoft... and anybody who seemed like a likely candidate to build something like Steam."
"We went around to everybody and asked 'Are you guys doing anything like this?' And everyone was like 'That's a million miles in the future... We can't help you.'"
Valve began developing Steam in 2002 and the platform launched in 2003 with as many as 300,000 gamers testing the service during its beta. As of 2015, purchases through Steam totaled more than $3.5 billion with 12.5 million concurrent users and thousands of games.