I've always wondered why on earth parents would behave in such a way... WHY would you want your child to be "Gifted" (capital-G-gifted, I mean). Having lived this, honestly, I think I'd much rather have a kid who was 'bright, but not gifted' than PG. Besides, unless this is overtly delusional thinking, and what happens if you succeed in prepping your child into a gifted magnet (or whatever passes for the rarified differentiated instructional format locally)?? Won't it be pretty damaging for an otherwise bright, even above-average, youngster to try desperately to keep up with a pace and level of instruction that they simply aren't intended to cope with?? ??? I truly do NOT understand this in the context of loving parenting.
I think the relationship between IQ and success is monotonic -- the smarter the better. This does not mean, of course, that other qualities do not influence success or that the correlation between IQ and success, however defined, is very high. So it is not surprising that many parents want their children to be really smart (or good-looking, or athletic).
As for getting into a gifted program -- suppose the cut-off is 130, your child's "natural IQ" is 120 but he can be coached to a 130 score, and the alternative to the gifted program is a regular program where the average IQ is 100 and the curriculum is geared to students with IQ below that. I'd rather have my child in the gifted program.