fttc

I want to book some flights. I checked on Google and went to United and it's available there. It's just some regular priced tickets, not mistake fare or anything. So I went to the thankyou website thinking to redeem the points toward that. It's not available there, and whatever showed up takes like 30-40 hours. I thought the thankyou website's search engine is just wacky. So I called in to book over the phone. What I was told shocked me. I was told that the travel portal only purchase a limited number of tickets. Once there are gone, then they are gone, even if the airlines still sell the tickets on the same flights. I talked to multiple agents and got the same answer. So this now is just like the airline points? Seems that they can limit the redemption by limiting the availability.

Are people aware of this? Or am I just so un-informed that I just realized this?