They literally cannot. If they offer a game at a lower price than Steam on other storefronts, Valve delists the game from Steam, which publishers cannot afford to since it's the de facto leader of the industry, and forfeiting Steam sales means forfeiting a huge chunk of sales. Listing "they could just not sell on Steam" as a "trivial" way to get around Steam's monopoly is so willfully moronic on so many levels that I don't think I need to explain to you why that would be a bad idea.

As for "doing a different edition of the game for each platform", that's also a no-go. The content parity clause extends to DLC as well, and the link provided above by the other user includes multiple examples of games that were forced to match Steam's price on other storefronts despite not being compatible. One such example:

There are ample examples of Valve explaining and enforcing the PMFN.
Valve enforced the Valve PMFN, for example, to block competition from the Discord Store. As detailed below, Discord launched a competitor to the Steam Store that charged only a 10% commission. As Discord offered a much lower price, some publishers wanted to steer customers to Discord, where the publisher could charge a lower price to the customer while growing its own revenue.
In late 2018, for example, one publisher had been selling its game on the Steam Store for $5, but launched its game on the Discord Store (enabled for Discord’s gaming platform) for free. Valve detected that the publisher was charging different prices on the two storefronts, and told the publisher that offering its game for a lower price on Discord violated the Valve PMFN. Valve insisted the publisher renegotiate its deal with Discord and ensure that gamers buying the Discord version pay the same price as gamers buying the Steam version.
Valve’s enforcement of the Valve PMFN harmed Discord, publishers, and gamers. Discord was unable to use price to grow its share of the market. Publishers were unable to reap the benefit of Discord’s lower commissions. Gamers were denied the ability to purchase the game for a lower retail price.

Having a central store is nice and all, but I should not be forced to pay my games on GOG, Epic or whatever the same price that Valve charges on Steam. That doesn't benefit me in the slightest. Heck, if anyone else other than Valve was forcing their competitors to match their prices, the outcry from the gaming community would be huge, and justifiably so. But since it's Steam, nobody cares and "having a central store" is used as a smoke screen to cover their shitty monopolistic anti-consumer practices.

EDIT: My god, the Steam fanboys in this thread are insane. You can like the storefront and still criticize its anti-consumer practices. Your Lord and Saviour Gaben won't knock on your door to kiss you, no matter what you do.