Well, technically speaking they are not annuals but many of the commercial providers of these sometimes gorgeous-looking H. macrophylla mophead plants are sold in locations where they are not winter hardy. And if they are not winter hardy, they do not survive local winters when planted outside. So in essence, they become "annuals".

Another take on the term: the companies that supply these shrubs prefer (and will not complain) if you buy them, keep them while their blooms last, discard them when the blooms fade AAAND then buy another one. Ad infinitum as they say.

But with some of these shrubs, you can plant them outside and grow a full sized shrub. It probably helps if your climate is temperate. I have one that was purchased at a Kroger's store and it is doing fine. But how can anyone tell if they are winter hardy when they do not even provide the name of the variety or its hardiness zone in a plant label, right? Oh well. That is how they make their money.

If you ever buy one in Zone 7 or warmer, try planting them outside. Some may actually do fine in Z6 as well. For extra safety, winter protect it since they will normally be varieties that produce blooms from old wood. Old wood varieties develop invisible flower buds in July-August. But since these guys are forced to leaf out early, they may produce flower buds well before that on their first season. They will go back to normal cycles on year two.

Some florists also grow mini-hydrangeas, almost bonsai type things. I believe it's technically possible to bonsai a hydrangea, but I'd certainly wouldn't expect anything with a fragile stem and root system to survive outdoors in a deciduous environment.