@James *******
1) Every single person in Thailand can open a Bank account with agent help. It's again a very basic thing agent can do.
2) Bank account closed is with Bangkok Bank, and due to the scandal a year or two ago, about a bunch of Chinese getting scammed with those mule account. That's why 95% of the problems right now are with that specific bank.
3) The proof of 500k is not a requirement with an agent. It's the same with retirement, marriage, etc, people who by-pass the financial requirement either don't need to show that proof or agent will put money in your account for 10 minutes.
4) In brief, the agent is a facilitator, so you pay a premium price for the DTV extension (which is still not expensive), and your agent coordinate with immigration officer to make it happen quickly and safely. Stamp is real, etc, but officer get a cut. It's truly not complicated. it's the same with every single visa and extension. Bonus: You don't even need to leave your house! You send your passport by mail, and get it back by mail. This has been going on for decades.
5) Agent can do much more complex things then this. For example, you can be on 2 year overstay... and for a big sums, the agent can get rid of it and get you a visa without issue. So the key point that you are missing is that an agent is a facilitator and a problem solver. So you don't need an agent unless you don't respect every single requirement for said visa or extension.