It's important to point out that it takes a long time for uptake of new versions of ad SDKs. The general assumption is that it takes about 6 months after release of a new version for 50% of ad traffic to come from that version or newer. Also, for every version you release, approximately 1% of traffic will never upgrade past that version.
In that kind of world, over-collecting data makes sense, especially if you think nobody will ever find out. Like total / and free disk space. There's no good reason to need those, right? But let's say an advertiser comes to you and says "we want to spend $1M / day to advertise our 10GB game, but only to devices that could install it." All of a sudden it's useful to know that a device only has 8GB of disk space, or only 100MB of free space.
So OK, if we didn't collect disk space, now it makes sense to collect disk space. Let's add it to the SDK. It takes a month or two to release a new version of the SDK. 3 months to get any meaningful traffic from it, and another 3 months to get up to 50% of your traffic. Assuming the ramps are linear, 4 months of 0%, and then 3 months of ramping to 50%, 30 days per month, you'll make $22.5M in the first 7 months. But if you had the logic in there to begin with, you'd have made $210M during the same time period. That makes it an easy choice for the business folks.
There are answers to this, but they all have drawbacks. You could limit data that ad agencies can collect. This reduces the value of ads. And agencies have learned that some data (like location) is low-value and high-risk, so they're removing the ability to supply it. I think it'd be better to support a model where ad code can be updated independently of the app. This way we could push out bug fixes faster, and could remove our just-in-case collection, but Apple has no signs that this is coming soon, and Google's answer has been such a shit-show that we aren't considering it viable over the next 4 years.
Edit: To address screen brightness specifically, it's a very rough proxy for age of the user.