Storks bring 'em.
So anyway let's talk about this more generally. Probably because the environment changes all the time, glaciers, meteor strikes, Republican leadership of Congress, etc. At the same time you need insurance, a fall-back mechanism whereby you can cut your losses and get out of an experimental adaptation that isn't working. But once the body is built, it's hard for it to learn really new tricks to deal with major environmental changes, as alas we all learn as we get older. So every few decades the molecules design and build a new body and move into it.