Some tasks are serializable other tasks are sequential.

A lot of game dev tasks are sequential, the code to do thing X has to be developed before feature Y can be implemented. And you can only have one dev working on one feature at a time, even version control has its limits.

Super, a super simple task you can throw more people at.

The cake one I like because it highlights that this is a complex task where more people are going to get in the way. If you throw 5 more people into baking a cake halfway through it's going to take 5 times as long because now you have to explain to them what you were doing, what your system is, how they should work together as a team. The overhead of adding them isn't worth it, and you'd probably just say "go stand over there I'll finish it"

Now, if you knew ahead of time you had an order for 50 cakes and you were given 5 people, now that's a completely different problem to solve. You now have a leader, tell this person is on mixing, this person is on frosting, etc etc.

So, in short, it's clear you aren't an engineer, and it's extremely clear you don't know what a strawman is

They could, but they wouldn't, they would organize their work to be more productive and achieve the stone moving faster.

Paraphrasing Proudhon's What Is Property :

The capitalist, it is said, paid for the workers' days; to be exact, it must be said that the capitalist paid as much 'one day of work' that he employed workers every day, which is not at all the same thing. Because the capitalist did not pay for this immense force which results from the union and harmony of workers, and the convergence and simultaneity of their effort. Two hundred grenadiers in a few hours erected the obelisk of Luqsor on its base; one cannot assume that a single man, in two hundred days, would have achieved the same feat.

Hiring 50 people to work on a project fundamentally changes the way the project is led compared to how it was by a single person, suddenly you will need additional workers to plan for the work to be done, to organize each task, to keep track of what's being done, and whatnot. Howard claimed there were 450 people working at Bethesda, and look at the state of Starfield ; Larian Studios allegedly grew from 100 to 400 people, and look at the state of Baldur's Gate 3 ; ZA/UM reported laying off 25% of their workforce, implying they were a team of 100, and look at the state of Disco Elysium (on release and/or Director's Cut, the ongoing legal battle is a whole other subject), it's a lot more complex that just numbers