Is this a trick question? I promise the reason people have been using page builders for at least the last five years is not because of their "cool" (pre-block) modules.

It's been to manage layout. The modules are convenient. And many of them are truly clever. But even genuinely bad page builders (and even ridiculous alternatives like ACF Flexible Content fields) can at least lay out a set of columns that
a) adjust themselves accordingly on different screens without supplemental CSS
b) handle other annoyances like equal-heights columns and vertical centering.

All the better page builders (even shortcode-circus ones like Divi and Visual Composer these days) will let you edit in situ, set colors, font sizes, spacing, margins, background images, apply responsive hinting, preview and tweak in tablet and phone screen sizes, set column stacking, and any number of other things most people want to do. Without writing acres of CSS. And especially without doing the silly edit/preview/edit/preview cycle that we had to do with old WordPress and still have to do with Blocks.

Speaking of edit/preview and CSS, the other day on my local WP community slack channel someone demoed a two-column block, divided evenly, with text on one side and a video on the other. The overlapping was embarrassingly bad. It's shocking that core container blocks can't even handle margins properly without user-added CSS.

I've said repeatedly over the years that page builder were the tech equivalent of a blister or callus caused by not being able to do simple layouts in core. That's still true.

So to answer your question: yeah, yeah, there needs to be at least a grid system in core. There really needs to be a layout,

Gutenberg architecture evidently makes it painfully difficult to incorporate blocks into any of the modern page builders. This is not a condemnation of page builders, it's a condemnation of block architecture. Maybe with their round of venture capital Elementor will be able to make it work. I dunno.

But one way or another, maybe 15% of all web pages use much more than
- Text boxes
- Photos
- Headers
- The occasional gallery, form, product, or other widget-y thing.

But 100% of them need to be laid out, preferably taking less time to do so than it takes to add the actual content. Preferably while adding the actual content. Blocks and the block editor don't do that.

A lot of us are pretty tired of hearing people say "well, Weebly/Wix/whatever let me do XYZ." When they say things like that it's pretty much always about layout and design. Because that's usually the only thing those tools are good at. And unfortunately it's one of the big things users care about.

So yeah, the block editor needs a grid editor. In core. It needs one just to get into the game.