To me, it seems like the 4a 5G is the better phone (slightly larger screen, audio jack!!!) and is cheaper by a couple of 100 dollars. Is the wireless charging and water-proof worth the extra 200?
It also looks like the 4a 5G has two sub-models for different networks, whereas the 5 has them all in one model. So if you desire more portability between 5G networks, the 5 is a better option there.
That's pretty much all I could see as notable differences. Not saying any of that is worth the price difference, but perhaps it explains it a bit. I am definitely going to wait for the inevitable Black Friday deals they do shortly after release. They discounted $200 in past years, which makes the 5 much more palatable to me.
After a while of use, some of those (more RAM/battery/screen/body material) are not tangible changes you notice. But, yeah, am with you on needing separate devices for different networks and that maybe 5a might be the right value-for-money.
Effectively you're just paying for 90hz screen, 2GB+ RAM, IP68 rating, wireless charging, but also losing the 3.5mm headphone jack and a bit screen real estate. Same storage (128GB) and SOC (765G).
One thing to remember is that these Pixel phones are likely produced by different teams at Google. The flagship phones are made by the US team, whereas the *a-series made by the former HTC team based in Taiwan (which Google acquired a few years ago). Latter has more experience with phone-making so perhaps unsurprising the outcome.