and other online stores have rarely offered features compelling enough to lure people away.

The fear among some developers is that doing so can lead to penalties or even expulsion from Steam — a potentially devastating outcome for their game sales.

The US lawsuit Newell was deposed in, which has been certified as a class action, alleges that it “is not economically feasible” for game makers to leave Steam in favor of a rival store and that they are effectively “forced to comply” with Valve’s rules and high fees.

The above quotes all point to exactly what I've been saying: Other stores/platforms simply don't appeal to users.

“Customers have enormous choice,” Newell has testified. They can decide “where they purchase their products, whether they buy the game on an Xbox, whether they buy it on Steam, whether they buy it on Epic Games Store or whether they buy it directly from software developers.”

No one is forcing anyone to use Steam.

EA experimented with everything including opening its own PC store and stopping major releases on Valve’s marketplace, only to reverse course and eventually bring big-name titles such as The Sims 4 and Battlefield V back to Steam.

Competing marketplaces, meanwhile, have failed to match even Steam’s basic capabilities, never mind its emotional resonance with users. EA’s original store was filled with glitches and had nowhere near the number of third-party titles as Steam, while Epic’s rival launched without such standard features as user reviews and a shopping cart for purchasing multiple games in a single transaction.

They tried, and failed, to launch BFV and Sims on just their own platform because IT WAS A BUGGY MESS. Not because StEaM hAs A BiG MaRkEt ShArE.

Not even having a Cart in your online store is mind boggling. Also, if I have to go to Steam to see reviews I might as well buy it there.

In 2017, Kassidy Gerber, who works in business development at Valve, wrote to Warner Bros. executives that preorders for its new Middle-earth: Shadow of War game had been deleted from Steam because the price was “significantly higher than what was available at other retailers for the same version of the game.”

So Valve protected their own brand by not wanting to be associated with "significantly higher prices". Sounds like a sound business decision to me.