I don't know if anyone else takes it this way, but to me R2-D2 and C-3PO are character names, just like Luke and Leia, but they are also model numbers, like T-16 or ARC-170. So although we associate R2-D2 as a character, I always figured there were thousands of R2-D2 units all over the galaxy, and its only the unique experiences of this particular R2-D2 that make it part of the story. I guess its a lot like some people treat their car, almost like it has a personality. And then once you sell that car, everytime you see that same model and color, you almost think its your old car. So its almost no surprise that Owen doesn't try to see if this is the same R2-D2 unit that he'd met once for a couple days 25 years prior. Even on a farm planet, I'm sure he's run across a handful of other R2-D2s in the meantime. Plus its rather clear, based on his treatment of Luke, that he doesn't look back on his interactions with Anakin with fondness.