If I had to guess, they place them where they think most consumers will use them. Many items from Home Depot and weekly grocery trips aren't conducive to bicycling. You might try writing a letter to the city planning department and suggesting that they require more bike racks at the commercial centers. It won't help for existing locations, but will for future ones.
Also, keep in mind that the more vocal Irvine residents don't like "the other" "invading" their community (see, e.g., affordable housing, light rail, etc.) and bike racks would allow those who can't afford cars to work in the community.