Answer: I suspect the midsummer starting is traditional for many perennials because that is when the spring garden rush is over and yet the remainder of the growing season is still long enough to grow the plants large enough to transplant safely. The soil temperature is also warmer in midsummer which in some cases speeds germination. While you can certainly start them sooner, and many people do, it will not give you blooms any sooner than those started in midsummer.
You could also start them in their final locations, but this may cause you some problems. First off, germination can be quite erratic, and the first year plants will be very small and will not take up their allotted "mature" spacing. This may look a bit odd. Secondly, growing them in a special area of their own allows you to keep an eye on them and give them any needed extra care such as more frequent watering than their mature relatives would require. Finally, many perennial seedlings grow more slowly than you might expect, and it is really helpful to know exactly where they are for the first few months. More gardeners than I can count, myself included, have accidently pulled up treasured baby perennials in an absent-minded weeding session!
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