Because a lot of people who use agents don’t actually meet the criteria for their extensions, and some people who meet the criteria just couldn’t be bothered to do it themselves.
If your license in your home country has expired, and you have a current 5 year Thai driving license, you are eligible to apply for a Thai IDP, but you will require a residency certificate from your local immigration office.
You can extend your 30 day visa exempt stamp for another 30 days at your local immigration office, will cost you 1,900 baht for the privilege. But it is a once off on your current 30 day visa exempt stamp.