Maypole - how do you judge what is 'lesser' in education though?
As I put in an earlier post, reception is far more critical to a child's success in the future than say, Year 4. Not because Year 4 isn't valuable, but because if a child fails to learn to read then they will fail through the rest of their school career, and this is taught in reception.
If you can't identify the 'lesser case' equivalents, then where do teachers start?
As others have said, you get your qualification by yes, doing some work at uni on the theory of child development etc, but you also get it by sitting in a classroom for short periods, to begin with, which are monitored and critiqued by tutors and/or the class teacher. Those periods are increased in length, until you are teaching for a few weeks at a time - so by the time you get your bit of paper you have stood in front of a group of kids loads, with experienced teachers telling you where you went wrong....
Finally, your training will mean you are specialised in a particular area. So if you specialise in early years, you will be taught how to teach reading. You will, on the other hand, be taught how to, say, manage a classroom of kids about to turn on bunsen burners for the first time if you choose to specialise in secondary chemistry. These things don't have much in common. So if you want to start people in reception to gain experience, it actually won't help them at all when they get to secondary and have to deal with kids who already know how to read but who need to have chemistry made understandable, fun and safe. It's a completely different skill set, and a completely different knowledge base. Why not start someone at the level at which they will continue, and make sure they have all the relevant knowledge, and have practised the craft at that level under supervision by someone else who has practised the craft at that level?
Otherwise what you are saying is 'Great, you've proved you are good at teaching a five year old how to recognise his own name, so now we want you in a secondary school and you'll need to teach the sixteen year olds how to build an electric circuit. Off you go, and don't worry, you've taught before so you'll be fine.'