It's a common practice, a problem with authentic child actors is they generally change once the original actor hits puberty, so higher pitched adult voices are common.
One of the behind-the-scenes videos on a Ed Edd n Eddy disc had some funny stories of the voice actors meeting fans (e.g. Ed's VA spotting some kid watching the show on a TV while on vacation in Jamaica and throwing in a "huh huah, thut guy is funneh..") and Kevin's VA had to keep doing their voice since kids didn't believe her whenever she mentioned it.
Money is the root of the love of money.