Because an uncapped plan is one leaked session away from an unbounded bill, and the honest way to price that risk is a ceiling rather than a higher price for everyone. 150 a month is five a day sustained, which is more than anyone researching their own ideas gets through.
The separate 20-a-day ceiling exists for the same reason at a shorter timescale: 150 reports in one afternoon is a script, not a customer.
Every report costs us real money to run — model calls and a slice of Apple Search Ads quota — which is why it is metered at all rather than unlimited.