It isn't. It also isn't the only way to do a general release splatbook with new player options.
I think there are ideas and formats. I just don't think "the big book of splat" is a desirable theme or format.
For one thing, they just started playtesting new feats a while back.
There's maybe four pages of feats in UA. I have no doubt they're testing some feats, but it need not be a long section.
There is only 6 pages of feats in the PHB. We don't need to double feat options.
For another, race and class feature variants can take up to half a page, easily, if they include story elements.
Subclasses can can easily take up a full page, especially if each one has its own art. Same deal with sub races.
I can't think of many sub races that would take up a full page. In many cases, the entire mechanics of a race - both race and subclass - fit on a single page. Half a page and art could be doable, but art is expensive. Possibly the most expensive part of the game. Art every page would be almost cost restrictive.
All the subclasses in the PHB fit into 30-odd pages. You could double the number of subclasses in the PHB with a similar number of pages, let alone 40 or 50.
And then, we get to discussion of different ways to use those options, DM material, new items, and optional rules modules like Organization rules, etc.
There is no reason a splat needs to have even a dozen new subclasses. Nor would a few dozen obviate the usefulness of more later. The edition isn't "done" just because a given person won't use everything published for it. I'll never use anything for the wizard, cleric, and probably fighter or sorcerer. I won't use much of what's out there for the Druid.
Most folks i know only use half the classes, at most, over a half dozen years of playing an edition, and don't use options for classes they like unless they like those options. Thus, new material is quite nice.
I'm not saying "no new subclasses ever!" I just don't want the pure mechanical, no fluff book that seems to be so desired on these boards.
All in all, SCAG has a 60-page section for new rules (with a lot of fluff) which seems to be a good size.
I've pitched the idea of a Hacker's Guide a few times: optional rules, more advice on optional rules, monster customization rules, more advice on making subclasses, races, and subraces.
Plus it could draw examples drawing from campaign settings from the past as a sneaky way of updating that content. Use it as an excuse for a warforged and kender, for defiling and moon magic, for the artificer and knight of Solamnia, and other campaign customizations. It can use the ranger as a prolonged example, which is a good way to not only include a revised version of the class (without just being "here's a new ranger") but also include a non-magic variant as well. And it can even sell the DMsGuild, possibly reprinting some content from that as examples of fan design. Encouraging people to check out the website.
In short, crunch, optional rules for DMs, and advice. The big book of beginner design.