Notably, anywhere in the AZ is simply a launch constraint, and the hyperscaler can and does place you in the most advantageous way for them which matches against the 'customer promise' (constraint). They in practice do model both the volume and cost of East-West traffic in their network topology and will wherever possible not place Instance A and Instance B all that far apart in the AZ, particularly if that AZ is one of the big ones with 20+ datacenters worth of capacity.
All the other points you made are also excellent.
Some of that is capex, and some is opex. But it's worth noting that the hyperscalers are doing something far, for more complex than almost any single-tenant physical installation would do.
In a hyperscaler cloud, I can run an instance anywhere in the AZ, and I can connect it to my VPC, and I get what appears to be, functions like, and performs as if I'm plugged in to conventional network infrastructure. This is amazing, and the clouds don't even charge for this service as such. There are papers written about how this works, presentations are given, costs are bragged about, and entire industries of software-defined networking are devoted to enabling use cases like this.
But an on-prem or normal datacenter installation doesn't need any of this. Your favorite networking equipment vendor will happily sell you a single switch with tens of Tbps of switching bankwidth and individual links that are pushing 1Tbps. It takes maybe 2RU. If you are serving 1 billion DAU, you can serve up a respectable amount of content on that one switch. You could stick all the content in one rack (good luck at that scale), or you can scale across a few racks, and you don't need any of the amazing things that hyperscalers use.
And, realistically, very very few companies will ever need to scale past that in a single datacenter -- there aren't a whole lot of places were even 100% market penetration can find 1 billion people all close enough to a single datacenter that it makes sense.
So the apples-to-oranges comparison cuts both ways. You really can get a lot of mileage out of just plugging some off-the-shelf hardware into a transit port or two.