Because, if you are a company that uses AWS at scale (deploying literally thousands of resources at a time), you care more about meeting demand and getting resources to spec, than you do about the cost of an individual elastic IP...
The price of each individual resource isn't something that you want to see on every screen you touch. It literally clutters the console with information that you couldn't give a shit about when your company is making millions (or billions) on the services you provide. You care more about your service's reliability/scalability/uptime/etc than anything else. This is priority numero uno.
If and when cost analysis becomes priority, you look to see where you may be overprovisioning resources - hence, the billing console.
But for the larger players on AWS (the multi-billion dollar ones that AWS cares more about making happy than you or I), an extra $100k in AWS expenses in a year isn't a worry, it's a write-off.
I agree, having set up serverless and lambda for an api/app that was to be used a few times per day, the billing made no sense at all, it would increase even though the services were used less and was difficult to find what the costs were or whether they could be reduced, eventually I had to shut it down and create a non-lambda solution somewhere else because I had little control of the cost.
AWS billing IS complicated but could be made so much simpler...