They bill that way because that’s the business model the telco’s were used to.
Mobile networks based their billing scheme on the long distance plans of old which charged based on minutes used, so usage based was the standard. You’ll note that they often still charge by the minute for calls and long distance depending on your plan.
ISPs meanwhile typically charge a flat fee like cable services, with different tiers based on performance.
DSL providers (telcos) had to use the flat fee model to compete with the cable ISPs, because customers had no interest in paying by the Megabyte when you can pay a reasonable flat fee per month.
Mobile providers meanwhile have no such competition, so they are free to collude and gouge their customers.
Some ISPs do charge overage fees for heavy users, but depending where you are located these are falling out of favor or even becoming illegal.
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