The basic configuration still ships with 8GB of RAM. 16GB is $200 extra. I had an 8GB MacBook Pro in 2013. It wasn't enough then, it's absolutely inadequate now.
I'm using my 2016 MBP with 8 GB and have never had a problem except when trying to run multiple VM's.
So 8 GB doesn't seem unreasonable as a minimum. You can upgrade if you need the extra, but a large number of buyers don't need it and prefer a cheaper base price.
The pro line has always been defined by CPU and graphics/display capabilities.
Memory is there to purchase if you're in the subset that has memory-intensive workloads.