Because the welding/bolting/etc joining processes are intricate and time consuming, and also equally as hard to repair.

A casting isn't weaker. You can make much more complex shapes with castings. You have much more variety in the thickness on the right points and interduce structural cross members.

See: https://electrek.co/wp-content/uploads/sites/3/2020/04/Tesla...

There is a reason Tesla cars do so well on safety while been pretty light comparatively.

> Why can't the blocks be modular and replaceable?

Everything is possible. But if you have many independent pieces that are all independently bonded that introduces a whole lot of complexity. Each connection is an additional place for failure. There is a reason every single car welds these pieces together. Its just not smart to try to use fasteners to connect these pieces. Its much harder to design, much harder (and more expensive) to produce, much error prone its likely also heavier.

What incentive does anyone ever to have pass on savings efficiencies? The answer is competition / competing goods.