So that game developers don't have to rebuild all the rules for their game universes for every game, they use Unity which is one of several products that offers pre-built frameworks to build their games on top of.

This offers several advantages, the main ones being:

1. You don't reinvent the wheel or the physics for angular momentum for every game, so development time is massively reduced.
2. You only develop your game once, but can compile it for use on many platforms. So a single developer can be writing code that can be used for the PC, Play Station, XBox and so on.
3. Skills learned in one game's development transfer to quicker development in your next game.

The way that Unity were paid historically was that you paid a subscription for each developer that was using it to write your game. There were several tiers of subscription, that met the needs of developers in small indies right through to huge multinationals.

It didn't matter how well your game sold, your cost was limited to the subscriptions you paid for. And you released your game bundled with the framework's 'run-time' from Unity that supported your in-game universe.

This changed recently when the executives at Unity had the spiffing idea of charging 20 US cents per installation of the run-time too, while also killing off the cheapest tier of the model subscription.

This meant that the indies suddenly have to pay more for each subscription, and they get to pay a fee for every installation of their game - not every sale - every installation. So every pirated copy - extra charge, every second install on the Steam Deck - extra charge...

What extra has Unity done to deserve the extra cash? In a nutshell - nothing. They just decided to unilaterally change their terms and take a bigger chunk of the pie for doing nothing more at the expense of the customers making the mistake of building on top of their product and being tied into their ecosystem.

As an analogy, imagine buying a season ticket to travel on the train to work each day, you pay €200/month for unlimited travel between the station closest to home and closest to work. This carries on for several years, and then suddenly the rail operator announces a 'communal rolling stock fee', every time you use your ticket there's a €1/passenger fee for each passenger boarding the same train as you whenever you travel! The more busy the train, the bigger the amount you have to pay!

What was a fixed monthly fee could be anything, you have no way to budget the cost, and you have to trust the same people screwing you to get the count right each time with no way of being certain they got it right...

What extra work is the rail operator doing, nothing they are just charging more for the same service.

And what did you do to deserve this? Nothing you just trusted the company you were dealing with to remain reasonable, and not invent bullshit charges.

Back to Unity...

As a result a lot of developers in late stage development plan to switch to a competitor for their next game. Those in early stages are looking at starting over. And there have been reports of publishers walking away from deals with developers because of the unknown risks of a new game because it was developed using Unity.

The easily predictable end result: many if not all of Unity's customers are vocally incandescent with justified rage.

TLDR: Software library company got greedy; they shat in the pool and tried to charge their customers a 'poop in the pool' fee, and every last one of their customers is very (very) loudly incredulous at such ass-hattery.