Some are, sure. But a lot are imported due to growing seasons. It's not always practical to grow everything in a greenhouse. Water, lighting, and heating get expensive. You can't insulate a greenhouse well because thet means less light.
Flower shops get shipped long waxed boxes packed with flowers end to end in order to do flower arrangements. Some flowers are also really prone to mold and stem breakage.
For example alstromaria, carnations, mums, ferns, baby's breath, saladago, and salal all have great shelf life. Stay for 2 weeks to 3 months in a cooler.
But plants such as lilly of the valley have a short growing season. Gerbera Daisy is so fragile and actually are shipped with plastic straws.
There really isn't a good way to bypass it. Flowers only grow when a plant is mature enough to produce seeds. From a gardening perspective it takes about 95-110 days for a tomato to go from seed to flowering/fruit. So every flower in your arrangement takes 3 months of planning from the seed.
Clarified with a long reply on theirs. Feel free to ask any questions. It's an interesting industry.