@Andrew ********
there's no such thing as a tourist visa on arrival.
If you arrive without a visa you can receive a 30 day visa exempt stamp. It's NOT visa. It's in the name visa exempt, meaning exempt from needing a visa.
Why would you waste a day at immigration applying for an extension and pay more than the price of the visa for that extension when you can just get it all online before your trip?
It's also possible if you fly without a visa that your airline will not let you board your flight without proof of leaving Thailand within 30 days.