Truth be told, I never thought it mattered. What meaningful conversation was going to come from Obi-Wan mentioning that he knew the droids when he had no intention of telling Luke about his parents in the near future anyways? Was he going to say hi to the droids? For what? They're machines. Very few people in this franchise treat droids as anything other than machines and Obi-Wan really isn't a character that's ever been much different. Don't remember if it's in the film, but the
Revenge of the Sith novelization has him getting on Anakin for being too attatched to R2. I can't even remember any real conversation or interaction Obi-Wan ever had with C3P0
It's not even that Obi Wan should, or shouldn't remember them. It's the pointless decision to tell a story a full 30 years before ANH, and bring those same droids into the films unnecessarily.
There was no reason to have Anakin be the one to create Threepio. It's pretty stupid really. All 3 films could have been done rather easily without those droids, or that connection to Obi Wan at all.
Having to go back and say, oh well, Obi Wan forgot, or oh well, he did remember but acted like he didn't, that's so stupid. Simply don't write the damn droids into 3 movies that take place decades before your original stories and you don't have to explain anything.
It's poor writing and storytelling. Ironically, he created a million new droids, but couldn't think to come up with two different ones to replace his misguided humorous droid sidekicks?
An entire galaxy of planets, and Threepio is going to survive 3 full decades and then get shot out of an escape pod and somehow end up on the farm of the son of his creator? Really?