Because it'd be better to legislate that full time employees (even management) be granted a paid full day off for each 5 hours over 40 worked in a single week. That can either be paid out at the end of the pay period or accrued for later use. If a company has "unlimited" vacation, those days of overtime must be paid out by the end of the next pay period.
As much as I prefer not to have government involved in much of anything, it's far simpler and far easier to legislate than to attempt to unionize. Unions tend to under-serve their average and above average members and is often not worth it. In the end, most would be happy to be compensated for personal expense and mandated overtime.
The idea that the American government will step up and legislate these protections is a hopeful one, but not even remotely realistic. It's a much better use of the time of game developers to unionize now, demand better working conditions now, and strike now.
Convincing even a majority of the gaming developers alone to do so to a point where those companies will agree to accept a union is unlikely. Especially across international and even interstate boundaries. There are some states (I live in one), where you cannot be required to join a union as a term of employment.
The biggest issue is generally a side effect of compensation. If the level of compensation after 40 hours exceeds the benefit to the company (reduced time to completion) they will pay or not. When it's effectively free, they will push. When less than half are willing, you won't get movement to/from a union.
There are also already lots of laws regarding payment to non-management, blue collar mostly hourly emloyees. Programmers are an exception, more so if salaried. There could probably be enough support to reduce the exceptions so programmers and IT in general are covered.
As much as I prefer not to have government involved in much of anything, it's far simpler and far easier to legislate than to attempt to unionize. Unions tend to under-serve their average and above average members and is often not worth it. In the end, most would be happy to be compensated for personal expense and mandated overtime.