Things that cause growth are generally a very bad thing in biology, because they cause cancer. Look at a list of proteins key to cancer and you'll find tons of growth promoters, some of which have the word "growth" right there in the name (the G in EGFR, FGFR, VEGFR, IGFR).
Growth must be tightly regulated to not end up as cancer, so that's why it's not abundantly available.