There are many reasons, but look the wild plant: are they the same as the plant in your garden? Do the flower at same time?
As example: orchids (usually) likes humid climate, so they grow only on such climate (and many growth just on trees). If you want orchids on your home, you need to provide the climate they expect (so usually hot and wet).
You may also notice that many wild flowers tent do blow in spring, before the hot and driest days. Or in forest (and in humid parts). There are exceptions, like poppies, but I do not find them so beautiful to replace my flowers.
We like also more exotic flowers (so not used to our climate). In addition we tend to select plants for long blooming, or large flowers, not for resistance to dry places.
As you see, there are various reasons. But you can select species which requires not much watering, just the selection will be limited. Roses are pretty resistant (but they are bushes, so with long roots). I like very much many cactuses flowers (but they tend to bloom just for limited time, and they may not like my weather). People tends also to use more plants for the leaves (bushes, or in any case perennials, so with deeper roots, or also sedges and decorative grasses). Or you can select some other species, like poppies, sunflowers, etc.
So: few species are adapted to your climate, you have a limited choice, and people like something different than usual wild plant they see at border of streets.