Why do 150 kW chargers still taper so hard at 60%?

I do a lot of long trips and the pattern is always the same: my car pulls 140 kW up to about 55%, then drops off a cliff. By 70% I'm lucky to see 70 kW. I get the chemistry and thermal limits, but newer cars seem to hold higher curves for longer. Is this mostly a car issue or are older 150 kW stalls partly to blame? Also curious how roaming prices compare now that more networks are on one app. Anyone found a tariff that doesn't punish you for charging past 80% on a road trip?