Traveling without an onward fly ticket is just silly when the airline requires it considering how cheap onward fly tickets are.
Actually you can do it for free you just pay more to book a fully refundable flight which I’ve done before than cancel the flight when you get to Thailand.
When the check in desks tell you you need an onward ticket most people don’t want the bother and time of doing this as sometimes it can take 2 attempts to go through.
Its poor planning not to already have something planned in advance before the airline tells you you need it.