Some people might find it fair to pay taxes where they live.
Paying taxes in a home country where you don't live anymore feels unfair, you pay money for infrastructure you don't use, and don't pay for what you actually use.
Paying a fair share of what you use sounds more logical
You talk about a DTA between Thailand and Australia, but the world is much bigger than only those two countries, other countries have different DTA's, so what you say about DTA is only relevant for your home country, and not the rest of the world.
you obviously don't know that DTA can be complex and not always ine country gets everything.
They can agree to share, depending on type of income and also declare types of income taxable for both, then most times making the tax paid in one country deductable in the other, but your DTA might be very simple, but there are a lot more countries than Australia.
How do you live in Thailand for 25 years without a permanent home?