Not a stupid question at all!
You are right, most Thoroughbreds, certainly those aimed at a career on the Flat, are born in the early part of the year, with the majority in March/April. They can be born as early as January and as late as August/September but these late foals are in the minority.
If left to their own, natural, devices foals would not start appearing until the Spring grass was available and the mares would foal right through the summer.
The main reason is to do with maturity on the racetrack. As a 2yo you want as early a foaling date as possible so that your horse has a maturity advantage over his younger rivals. By the time they get to 3/4/5 most horses have finished growing so there is less of an advantage to be had.
To give you an idea of the foaling stats from 2011 (Britain and Ireland):
January - 206 foals born
February - 888 foals born
March - 1,263 foals born
April - 1,321 foals born
May - 689 foals born
June - 142 foals born
July - 24 foals born
August - 1 foal born