They really don't....well, I guess some people do. Lots of us call it Spirea thungergii 'Ogon' because we feel funny saying Mellow Yellow, which is, I guess, a trademark name. I remember there was a plant in one of the display gardens at Behnke's and it was more trouble than it was worth. We'd sell our 20 or 30 plants and then, for the rest of the season, have to explain to infatuated would be purchasers, why they couldn't buy the display plant. Most of us would have happily given it away so we wouldn't have to tell anyone else that they had to wait for next year.
And you know what? It's one of my favorite garish yellow foliage plants. It's gauzy and graceful, a light airy contrast to, for example, the Ilex cornuta 'Carissa' hedge it terminates nicely along Hickey Hill Road in the Asian Collection . I even like the tiny white flowers; they bloom with the earliest of spring flowering shrubs.
Hey, who am I kidding? I like colors, bright colors, colors that seem like they just wouldn't fit into a garden. They do though. I could do more than a week of yellow foliage; I bet I could do a month......but don't worry. I won't.