Maybe because he had not found any paperweight. He found a lexicon.
No, for the analogy to make sense he would have had to find a disassembled lexicon which he carefully pieced together into perfect working order, only so his illiterate mother could use it as a paperweight. Which is ridiculous.
Besides, your main argument in rationalizing this seems to be that he happened to find old parts that specifically worked only for the construction of a protocol droid. But, to paraphrase you from earlier in this thread, "That's just your personal opinion. You didn't observe it in the film." The only data we have from the film itself is Anakin telling Padme that he's building "a protocol droid, to help mom." He doesn't say "I happened to find these protocol droid parts and decided to throw it together just for the hell of it. Maybe he can do chores or something." As far as we know, he somehow specifically set out to built a protocol droid to help his mother, presumably with her ordinary day-to-day activities. Which, again, is absurd.
Subplot?! And why shouldn't he have written that? It doesn't hurt anything. The problem is that you (and/or RLM) seem to find a problem that a protocol droid can do chores. It's like hitting a strawman.
No, the problems with this particular subplot as written, are at least threefold. As I wrote earlier, "For one thing it's fanwankery, pure and simple. For another, it shrinks the universe and introduces a level of "convenience" that stretches credulity beyond the breaking point. Then of course, to top it off, there's the fact that a junk dealer's slave would have no need for a droid specifically designed for etiquette/protocol and translating over six million languages."
Like Leia and Luke being brothers. Oh, almost forgot. That's OT so it's okay.
Now who's hitting the strawman? ;) In my defence, although I'm rather a fan of ROTJ, nevertheless I've always contended that the Luke-Leia sibling relationship is absurdly stupid and fanwakery on nearly the same level as all the inbred universe-shrinking of the PT.
There's a good reason most of us OT fans/"PT bashers" consider ROTJ to be the weakest of the OT...