Because it’s not a normal situation, you’re taking an international trip to a country which is overwhelmingly still a cash society.
Plus the point of showing the cash is to show you have actual cash.
Anyone can fake a bank statement and immigration doesn’t have the time or resources to send an officer with you to an ATM to pull up your bank balance.
Not to mention they’re targeting higher value tourists.
To be honest, unless you’re from an ASEAN country, you should have to show you have considerably more than 20k in cash, especially when getting a 30-45 day visa exemption or visa on arrival.
Unless he’s got enough money to last the rest of his life in Thailand, you’re missing the part where he needs to have some kind of income while in Thailand
It’s all about money. Either you have a business you can operate from or in Thailand, have a job you can work in or from Thailand or have enough assets already to move to Thailand and retire.
That’s really all there is to it, unless she makes enough money from her business to support you.
Once you’ve sorted out your money situation. you just have to see which visa type makes the most sense given your age, finances and whether or not you’re willing to marry her.