Legal issues. We live in a lawsuit-happy time period, and if some fat, middle aged lady slapped on some lotion and got a gross, bubbly rash, she'd probably lose a dress size running so fast for the phone book to find a lawyer.
Honestly, laws have been put into place that specify what has to happen to guarantee a product is safe for public use. Public demand for a meticulous testing process is important as well.
Why can't we be happy with all the tried and tested cosmetics, sweets and tchotchkes we already have? The answer to that is the REAL mystery, but I'm going to bet my money on the fact that companies want to make as many new products as possible to stay fresh, new and competitive, since a sucker is born every minute. It'll be there as long as we as a society reward greed, excess, and being a savage, unscrupulous litigious person. People can't wait for the new OPI shade of nail polish, for the new mascara that'll TOTALLY make their lashes like, eight times as fat, so fat your head will bend in half, omgggg. You get my drift. You can make a killing off those folks, and companies know that.