It's absolutely about the long game for them. During the initial launch the amount of bonus bets and promos each book was running were just completely bonkers. Getting people hooked by giving them their first few hits for free is one of the oldest tricks.
This isnt meant to brag by any means, but I made 5k in my first year pretty easily, and that includes skipping most of baseball season, but so much of that was done from signup bonuses. I'm off to a really successful start to this year as well, but like I stated up thread, you just have to stick to the bonus offers, free bets, and boosted odds. If you can train yourself to do that, which it sounds like you have, then there is definitely money to be made.
I'm guessing that you and I aren't the typical sports gambling consumer, and they can afford to keep floating us freebies because the vast majority of people burn through those and then double down with a bunch of actual cash. It's all about patience and sticking to whatever game plan is successful for you.