Why do citizens of certain countries get a 90-day visa on arrival in Thailand while others, like the US, only receive 30 days?

The post questions the disparity in visa on arrival durations for travelers from different countries, noting that some countries receive a 90-day visa while US citizens only receive a 30-day visa. The author suggests that extending the visa might require a fee (specifically referencing a 1900 Thai Baht charge) and is seeking clarification on the reasoning behind these differing regulations.