I have been flying in and out of Thailand for 13 years. I have family, property, and bank accounts in Thailand. I have never overstayed. I have been refused check-in for the first time. I am a foreign aid worker and often don't know my departure dates until a few days or a week before departure – hence often no outbound ticket. I've never had a problem with Thailand immigration. I was fortunately able to make a booking which satisfied the airline, and I could board, then cancelled booking the next day. Is there a better way to manage this?
Since the government has started cracking down on folks who misuse the system, airlines have become more careful of their responsibility.
Just about every Thai visa issued to foreigners has a time component, a limit on how many days you can stay (which I assume you know since you mentioned never overstaying). Without proof of exiting the country before this deadline, airlines are required by law to prohibit boarding. If the airline allows you to travel there and then you are denied entry, the airline can get fined and have to fly you home at their expense. In years past the government was lax about the rules, so the airlines were lax, but now the government is being more strict.
Probably your best course of action is to buy a fully refundable outbound ticket and then cash it in or change the date once you know your real date.
Another option might be to buy a cheap round trip (it sounds like you are buying one ways, which often are more expensive than discounted R/T), then not use the return leg or perhaps change the date, paying the change fees (which may still be a cheaper option than two one way tickets).