The wood is glued together (called edge gluing). Biscuits, dowels,
splines etc. provide assistance in aligning the pieces and add some
strength, but its the glue that does the job. A biscuit is one of the
fastest and more reliable ways to add this alignment assistance.

Wider pieces of lumber are expensive and less available. Also, if you
use power tools for milling the lumber, width capacity translates to
more expense in the tool (initial cost plus ongoing blade replacement).
The primary culprit here is the power jointer. blade prices go up
exponentially with increases in width.

For furniture purposes, a glued board is as strong as a single piece of
wood. In fact the glued joint is usually considered stronger the wood.
Bob