Because Epic and Tencent should have first pick from the IP farm.

Why is Valve being sued for almost $900 million

"The legal action, originally filed in 2024 by digital rights campaigner Vicki Shotbolt"

Vicki is a leading campaigner for children’s digital rights, with over 20 years of senior leadership experience in national charities. She is the founder and CEO of Parent Zone, an organisation that works with families and global brands to improve the lives of children in today’s digital world.

(Source: https://steamyouoweus.co.uk/about-us/)

That is why Valve is being sued for 900 million. Because Vicki Shotbolt wanted to. Why did she want to? Here is her claim (in her own words, not mine):

But Steam’s prices appear to be the lowest?

Steam can offer the lowest prices because of the anti-competitive price restrictions that Valve often imposes on game developers and producers (the Price Parity Obligations). This means a publisher or developer would not be able to list a game on another platform as well as Steam, unless the prices offered on Steam is the same or lower. This applies to games on all other distribution stores (including online and physical stores) not just those distributed by Steam Keys. This allows Valve to maintain the monopoly position it has for PC Games as there is not real incentive for gamers to go elsewhere where a game may be cheaper (which would then in turn enable those other platforms to improve).

It is also not possible to offer add-on content on other distribution platforms for cheaper or at an earlier time: this limits the ability of rivals to compete on price and enables Valve to charge the consumer higher prices in the absence of competition. The claim argues that the add-on content is a separate product, and that through the price restrictions and inability to purchase add-on content from another distribution platform or the developer itself Valve has illegally tied these products and limited consumer choice. Consumers must then purchase via Steam and pay its commission charge.

In the UK, dominant companies are not allowed to charge excessive prices. The claim argues that Valve’s commission rate of up to 30% is excessive given: competitors lower commission rates; the way the platform operates for the consumer; and the high level of profit that Valve is making absent a viable competitor (which its behaviour directly restricts as developers are not permitted to list games at lower prices on competing platforms). This unfair commission charge is paid for by the consumer.

"[...] but Epic Games wasn't sued when they bought Rocket League and Fall Guys to remove them from steam?

Steam has a much easier claim to be considered a monopoly. It's a little like (note: I never said it's exactly like or it is very much like—I only said it's a little like) Chrome being a monopoly for web browsers—everyone chooses to install chrome on their computers when they install a PC and prefer not to use the pre-installed Edge or Safari. Very few people install Epic games, much like very few people install Firefox. If you want to game on PC, you pretty much have to install Steam to play with your friends you know? Otherwise you're kinda lame and don't have friends.

Same reason nobody, not even Sony, sued M$ for buying Activision-King and Bethesda.