OTB, you're only working those stats for the Flat, yes? In that case, yes, obviously there's a much bigger percentage of animals in the lower divisions, where some are still working their way into the handicaps (post third-run 2 and 3 y.o.'s) and some are dropping back through the ratings as they age. Now that you have Dundalk in Ireland (an AW facility) you will probably see many more older horses retained in training, as we have in the UK, which bolsters the numbers of the lower-rated groups, regardless of their age definitions (3+, 4+, 5+). We have loads - shedloads - of 6+ animals running technically on 'the Flat', but they're almost all performing the most on the AW, where the standard surface is much kinder to older joints, as it is to younger ones.
As is the case with any demographic, the elite is, unsurpisingly, the elite, and therefore will always form a small percentage of whatever it is you're analysing - men's clothing, restaurants, social groups, cars - I'm sure there are more Fords and Vauxhalls on the roads than Bugattis, for example? The majority of Flat horse ownership, taking out the sheikhs, and in Ireland taking out Coolmore, is with multiply-owned or leased horses (such as partnerships, syndicates or racing clubs), followed by a small amount of business-sponsored horses, dual ownerships between friends, and then solo owners - meaning that the bulk of the type of ownership reflects the purchasing price of the animals, the trainers' costs, and the likely ratings their horses will attract.
In both the UK and Ireland, you have what is a tiny elite of ownership able to pay big bucks for higher-class animals, which obviously go to the minimum number of top training yards. The horses will be the produce of the highest end of the breeding spectrum and be entered up accordingly. If they fail to run well enough, they're disappeared - they rarely seem to just run downmarket for cheap 'n' cheerful owners, but they get sold to run out of sight overseas, or are just 'retired', one way or the other. That means that you do have a continual culling of the more elite-bred horses if they fail to match their price tags in performance - you really don't see these sliding down the ratings into Class 6 AW handicaps.
The horses which bulk out the lower echelons (Class 4, 5 and 6) tend to be those whose owners don't have deep pockets, per se, but who do like to keep a steady interest in horses which will turn out doing enough to keep them in training. Thus, having a win here and there in a Class 4 will be thrilling enough to keep the horse going, as will even lower Classes, since it is more enthusiasm about racing which maintains these animals, more than a coldly commercial eye to potential stud fees or filly breedability in future.
If there is a desire to clip out lower-rated animals by balloting them out frequently, then there will be a huge drop-off in ownership and you would, taken to its extreme view, end up with racing back in the hands of an extremely wealthy elite, from breeding through ownership and then directly back into breeding, in a self-maintaining cycle. There would then be a reduction in bread-and-butter runners, reducing field sizes and therefore, were they to regularly come in under 8 per race, seeing courses not being paid picture rights. It is not in the interest of courses or international betting to see racing cut back to where there are small fields and no doubt much reduced crowds, who won't be interested in compressed SPs.
There's almost the whiff of eugenics involved, where the assumption is that if you reduce the lower-rated animal, you will somehow miraculously end up with only high-class ones. As any breeder would tell those with such delusions, "if only it were that easy"! You'd only have to whip your multiply-winning mare to a Group 1 stallion and hey, presto! Instant success every time! Total La-la Land, of course, as any breeder knows. It's the old 'what if Einstein had a baby with Monroe?' question, where it's assumed that the babe would turn out to be a beautiful genius.
It's because the Einstein-Monroe doctrine doesn't work in real life, that you see card after card filled by Class 5 and 6 runners with excellent sires, but not trained by those who charge a king's ransom and certainly not owned by people who will have shelled out tens of thousands at the foal sales, when the little dear, bred to the back teeth, was first presented.
Chuck out this less able, but well bred and usually much loved, stratum, and say goodbye to a healthy betting medium, well-attended courses, thousands of people who can at last afford a real interest in racing, and all the knock-on effect through the industry, from less work for everyone - jockeys, farriers, transporters, stable staff...
In fact, if you want to throw a load of people out of their livelihoods, then go ahead!