Why can't you build one? I'm actually interested. It doesn't make sense to me why your mom wouldn't let you build one if 1) it's cheaper and 2) it's a fun experience
I can think of one really good reason: she's not confident that he would actually be able to build it. Assuming that this is a typical mother who doesn't know much about the construction of computers, she probably assumes it requires more technical knowledge of how computers work than she believes OP possesses. Basically, she isn't 100% confident he can do it. So if she bought him $600 worth of computer parts and he was unable to put it together, that'd be a grand waste of $600. Or, she'd have to pay even more money to have a professional assemble it, which is more money she doesn't want to spend.
Obviously this is fallacious thinking due to the fact that computer parts quite literally plug right into each other and require almost zero technical knowledge of electrical engineering to assemble. But I can still see why she'd play the "better safe than sorry" card.