I don't disagree it's a focus thing for many people. I'm often stunned at the lack of comprehension or attention to detail using any medium, even in person (also technical field).
Like look, I just said to do what you're asking would require 250 firewall rules...why are you now talking as if firewall rules aren't required? I even went through the simplest math out loud during this meeting, so everyone would understand how I came up with that number and didn't just pull it out of my ass.
People pay attention to what they want to pay attention to (or as my grandfather would say - people hear what they want to hear). If those questions aren't a high priority for their own work, they simply don't see them.
For OP: email is a terrible medium for such things, unless there's been a conversation about it, and this is part of moving a project forward. Anything out of left field isn't important to your audience, and... people dislike comitting to anything in email. As you work with people up the food chain, you'll find less and less happens via verifiable comms like email (which is archived).
That's why you have the phone call, to discuss it, and in closing state you'll send an email.