> If this were true then why have upper limits at all?
Because while the rulemaker believes that there is a range of potentially reasonable judgments based on particular circumstances, they do not believe that range is unbounded.
> The only reason I can think of is to protect large corporations.
The fixed minimum upper limit of $20 million is actually probably to prevent (or limit the effect of) large corporations using smaller subsidiaries and fancy accounting for GDPR-risky activities, rather than the upper limit protecting large corps.