I mostly don't eat whipped cream or anything similar. So on the rare occasions that I do eat it, I have absolutely no desire to make it myself. If I had a stand mixer, I might be more inclined, but I pretty much never want to whip anything that long by hand.
Also, I really don't use full-fat milk or cream when I cook, unless I really have to (and I often just don't make recipes that don't allow for substitutions). I don't care for the taste, and I don't care for the additional fat. I grew up drinking skim milk, and am far happier subbing that or yogurt or sour cream, when I can.
For desserts, I tend not to buy Cool Whip because I didn't grow up eating it, but I do sometimes substitute vanilla ice cream for whipped cream.
For people who don't want to sub ice cream (or for recipes where that isn't possible), Cool Whip is easier than actual whipped cream.