They also have the goodwill for helping make gaming on linux practical enough that there can be a handheld gaming console that runs linux (plus, of course, popularizing that entire new market segment).
They also don't constantly change large parts of Steam. Point is, people like that Valve is relatively stable, and that's why they insist on sticking to it. The industry is otherwise such a shitshow. The stability is an especially important value for a platform that holds, in many cases, thousands of dollars worth of games for people.
When you've tried the others and then been denied playing your game because of their crappiness... it gets old quick.
In most other sectors the bar is higher. They've earned their near-monopoly fair and square (unlike Apple, they don't do anything to block competing marketplaces - they're not even preinstalled) by being head and shoulders above their competitors in terms of quality - the fact that that's due to those competitors all sawing their own legs off doesn't change that.