well, OK, I'll bite.
I was super clever at school - top of the top set in a very selective grammar.
Then I spent my University years engaged in sex'n'drugs'rock'roll & came out with a rather embarrassingly crap degree. I worked in industry for several years before wandering into teaching - not my degree subject.
These days, they wouldn't accept me onto a teacher training programme with my Richard in the wrong subject, but somehow I blagged my way in in the mid 90s.
I was a rubbish teacher initially. I really, really knew my subject matter - I just had no idea how to communicate it to bottom set year 9, given my fuzzy memories of being taught it 20 years previously in a class of super keen clever biddable young ladies. I reckon it took me three years to become a decent all-round subject teacher, & it was the toughest learning curve I've ever experienced.
Starting out now, I'm pretty sure I'd be one of the 50% of trainee teachers who are gone, expense of training them wasted, in those first three years.
Despite my rubbish paper qualifications in my subject, no-one has ever been able to fault my subject knowledge - I invariably used to get 'outstanding' for that bit in observations & was very much acknowledged as the department smartarse. I do have a very genuine passion for my subject & am a proud 'lifelong learner'.
Overall, I'm a good teacher, with a track record in the UK of getting my students very creditable results.
Now? I'm teaching abroad, having been welcomed in to a promoted post with open arms. It's a great job for all sorts of reasons which I've talked about on here before - I appreciate it's not an option for everybody, but it's certainly been the most positive thing I've done in years.
My old job (at a very over-subscribed & successful UK school)? Still vacant.
The bright young NQT I mentored last year? Emailing me for contacts overseas so she can escape too. She has a very sensible medium term plan which is pretty much 'keep head down, get a couple of years' UK experience as still well regarded by international schools, head abroad with bf for much more lucrative teaching opportunities, live cheaply overseas & save for a house deposit for a couple of years before getting out of teaching altogether'.
So OP: a) no, being a bit of a cleverdick doesn't make you a fabulous teacher, necessarily; b) actually, not every successful teacher has 'top grades' in the subject they teach & c) as everyone else has said - there's a chuffing great recruitment & retention crisis in UK teaching.
Until the t&c & the culture change, you simply aren't going to be able to insist that potential teachers have 'top grades'.
You seem to be fondly assuming that the profession is crowded with amiable time-serving dimwits job-blocking a host of top graduates who are desperate to crack on & do the job properly.
You are very very wrong about that.