I think most of my opinions about this have been expressed in one way or another by others, but let me add my $0.02.

Briefly (I hope!), I think the source of your perplexity lies in two assumptions that appear to be implied by your comments:
(i) there is only one relevant, or perhaps only one dominant, source of enjoyment for people who race their boats, i.e. the pleasure of competition that compares nothing other than on-the-water sailor performance within a predefined set of parameters.
(ii) strict OD sailing is the only truly ligitimate way to compare sailor performance.

Taking the second point first, many would argue that for most practical purposes, strict OD is really an abstraction that exists relatively rarely (as in the case of factory supplied boats for a particular event), and that in practice the performance differences between different designs within a formula restriction are quite small enough to enable sailor performance to be compared equally well as in many nominally OD fleets, while also having benefits in cost and availability arising from competition between multiple manufacturers.

I think this is a credible argument - it is at least consistent with the growth of Formula racing, but it does itself involve an assumption that may be difficult to prove conclusively. On the other hand, I think there are flaws in the other (first) assumption that are rather more significant...

I don't think anyone who chooses not to race OD would deny that there is any enjoyment to be had in OD racing. It's just that it's not the whole story. There are at least four other sources of enjoyment that I can think of, and any of these could potentially lead someone away from sailing in a particular OD class or towards sailing in an OC fleet:

(i) the pleasure of a sailing a particular design. This includes preferences associated with differing performance, equipment and required sailing skills, for example.
(ii) the pleasure of being able to modify their boat in the way that (a) suits them and (b) they think will produce the best performance on the water. While it may mean nothing to you personally, for many people this is a major component of why they enjoy competing.
(iii) the pleasure of sailing in a bigger fleet than can be provided by OD racing within a reasonable distance.
(iv) the pleasure of sailing against people they consider to provide the best competition available.

Of course the situation is complicated further by the fact that there are likely to be different costs associated with some of these factors. But in any event, I don't think anyone should find it surprising (much less worth criticising) that different people consider each of these factors to be of different importance in their personal choices of how they spend they recreational time.

Not everyone is the same. That's ok. Isn't it?

Mark.