But they used to. Before Apple, every handset maker had to get any updates tested and blessed by the carriers. It was a royal PITA as you wanted to get everyone all of the bug fixes and feature enhancements, but the carriers had to take their sweet time certifying the updates.
Apple changed all of that.
And, in terms of your phone being aware whether the data path is wifi or cellular - yes, it matters less now, but there are many reasons why the cell phone should be aware (one of them is wifi calling - if you're moving into a wifi dead zone, you want the phone to be aware of that and switch your conversation to cellular so it doesn't drop).
Also, many people don't have great data plans, so they still need to be able to have their phones only do certain functions over wifi, to minimize their monthly usage.
Here's an example that the carriers wouldn't like: A school board has the policy set that all of their iPads automatically download any updates as soon as they're available. They provide iPads to all of their students. So, when an update comes available from Apple, about 250,000 iPads in that city all download the update as soon as it's available from Apple's servers. Depending on the size of the update, this could cause service impact to a lot of subscribers in that city if the updates were downloaded over cellular.
This will eventually change....as the networks become more robust, speeds increasing, minimizing download duration.
But, given Verizon and Sprint can't even seem to get their CDMA networks migrated off, I wouldn't be holding my breath for it to happen tomorrow.