The modularity is there with classic manufacturers too, just not "exposed" to the buyer. Cars use a common platform and you can still buy the classic or fastback, different styling packages, but you won't be able to self service them after the sale. I don't really know how much Slate is the Framework of the auto world, and how much it's just customization options at purchase time.

Most people who buy a car would never be bothered to "tweak" it later, upgrade, add stuff. Modularity also constrains the design and could add some reliability issues.

The biggest benefit would be home repairability so I think that's a big driver for why other manufacturers don't do it. EVs already require less maintenance so that's lost revenue.

P.S. Looking at the options on the site, other than the body style everything else is just as easy to have on any other car. Most of the customization is purely esthetic (wraps, decals, rim options, light plates) and even the practical options like light bars or roof racks are common in the OEM world for any classic brand.